‘Ismat’ literally means 'Protection ' In Shia terminology it means ‘A special Grace (Lutf) of Allah to a person which enables the said person to abstain from sins by his own free will.' Such a person is called 'Maasum.’
This Lutf (grace) does not make the Maasum person incapable of committing sin. He refrains from sins and mistakes by his own power and will.
Here is an example which will make its meaning clear: It is within the power of any person to go naked on the streets. (And, for that matter, in many 'civilized' countries women practise the 'art' of discarding all their clothes in packed halls
. But have you ever thought of doing so? No. Why? Because it is far below your dignity to behave like it. Mind it, you do not say that it is 'impossible' for you to do so. Surely, it is within your power, but you will never even imagine of doing so. Why? Because you think that such foolishness would tarnish your honour in your society.
Similarly, though a Maasum person (Prophet or Imam) has the ability to commit sin, but he never even imagines of doing so, because it is far below his dignity to stoop to the sins and mistakes
According to the Shia Ithna-asheri belief all Prophets and imams were Maasum, i.e., sinless and infallible; they could commit no sin— neither a capital sin nor a minor one; neither intentionally nor inadvertently; and this applies from the beginning of their lives up to their last breath.
So far as the Sunnis are concerned, they admit that the prophets could not tell a lie, neither intentionally nor by mistake, and they could not be Kafir whether before or after the declaration of the prophethood. As about other sins, their opinion is widely divided. Most of them say that the prophets could not commit other sins intentionally.
About committing capital sins unintentionally the majority says that it was possible, a minority holding that it was not possible about minor sins, they say that it was possible for the prophets to commit minor sins, though they could not commit such minor sins which would have degraded them in the people's eyes, like stealing a loaf of bread. Thus it is clear that the Sunnis have no clear idea about the Ismat of the prophets.

Why Ismat? Logical Reason
What are our reasons to believe that all prophets were Masum (Sinless)? The reason is very simple: God sent the prophets to lead their people onto right path. But this goal could not be achieved if the prophets had not been sinless.
Suppose there is a man who, just like ordinary people, commits sins or wrongs. And suppose such a man claims that he has been sent by God to lead his people to the right path and provide with his life a model of all that is good and virtuous in human nature. How can the people be sure that he is saying the truth, the whole truth and nothing but truth? He cannot command from his people the unreserved respect and undo table obedience which is his due. Therefore, to carry on the work of God, the prophets must have been sinless and immune from committing mistakes?

Proof From Qur'an
This logical reasoning is supported by the Holy Qur'an also. Look, for example, to the following verses:
1st Verse: God says: "We sent not an Apostle, but to be obeyed, in accordance with the Will of God." (Qur'an, Sura 4, Verse 64).
The prophets and the apostles were to be obeyed and followed, not that the followers were expected to check every action of the prophet to decide what was to be obeyed and what not to be obeyed. How could this be true unless the prophets and apostles are believed to be free from errors, sins and mistakes?
2nd Verse: God commands us to obey the Prophet: "O ye who believe! Obey God, and obey the Apostle." (Sura an-Nisa’, 4:59).
Again He says: "All who obey God and the Apostle......" (Sura an-Nisa’, 4:69).
In the same Sura it is asserted: "He who obeys the Apostle, obeys God." (Sura an-Nisa’, 4:80)
In all these verses of Qur'an, as well as in numerous other Verses, the obedience of God is made synonymous with the obedience of the prophet. Such assertion would have been impossible if the prophets were not Masum and infallible Just imagine what impossible situation would have been created if any prophet began exhorting his followers to commit a mistake or sin.
The wretched followers would have been condemned to the displeasure of God in any case. If they obeyed the prophet and committed that sin they disobeyed the command given by God and thus were disgraced. If, on the other hand, they disobeyed the prophet, they again disobeyed the command of God mentioned above about obeying the prophet. So, it appears that a non-Maasum prophet could bring nothing but disgrace and condemnation to his people.
3rd Verse: Add this verse to the above verses: "Do not obey those who do wrong."
Now the picture is complete. The prophets were to be obeyed the wrongdoers were not to be obeyed, the only conclusion is that the prophets were not wrongdoers.
4th Verse: Now comes the prayer of Prophet Ibrahim (a.s.) and its answer from God: "And remember when Abraham was tried by his Lord with certain commands, which he fulfilled: He said: I will make thee an Imam to the people. He pleaded: 'And also from my offspring.' God said: 'But my promise will not reach to the wrongdoers."(Sura al-Baqarah, 2:124)
This verse clearly shows that the promise of God to appoint Imams (This word means here religious leader, including both the prophet and Imam of Muslim terminology) will not reach the sinners.
This much will be sufficient to show that our belief is based on the clear understanding of Qur'an, concerning the authority and duty of the prophet towards God and towards his people.

Some Allegorical Verses
It should be mentioned here that there are some Verses in the Qur'an which give, to some minds, an impression that Prophet Adam (a.s.) and some other prophets committed some sins.
Qur'an itself declares that some of its Verses are clear ones which are the basis of the Book and others are allegorical. Then it goes on to say that the true interpretation of allegorical Verses is known only to Allah and to those who are firmly grounded in knowledge.
Therefore, it is necessary, first of all, to decide which verse has clear meaning, and which one is an allegorical, whose meaning must be decided upon in the light of the clear verses under guidance of the Prophet or Imam (who were firmly grounded in knowledge). Then, and then only, can we save our beliefs from conflicts.
To sum it up:
(1) Reason says that the Prophets should be sinless and infallible;
(2) Many verses of Qur'an support this view, as mentioned earlier;
(3) But still some other verses of Qur'an appear to attribute sins and wrongs to some prophets
What is a Muslim supposed to do? The old-established rule and practice demand that we must accept those verses, which are supported by reason, as the clear one. And as regards other verses (which show that they were sinner) we should seek what is their true and valid interpretation, within the framework of the Arabic language and grammar, as taught by the Prophet and Imams.

Did Adam Commit a Sin?
Let us look at the phrases used for the first man and the first Khalifa of Allah, i.e., Prophet Adam (a.s.). Qur'an describes in several places how he was advised by God not to go near a certain tree when he was in heaven; how Satan deceived Adam and his wife Hawwa through his false oath and how Hawwa, becoming convinced of his truth, persuaded Adam; and how he, inadvertently, ate from that tree, and then was sent to Earth.
This is the gist of those narratives. First of all, Qur'an clearly says that Adam was created as Khalifa of Allah for this Earth. Before being sent here, he was put in the paradise. It must be accepted that that paradise was not his permanent place of residence, because, even before his creation, he was destined to come to earth.
Secondly, according to Islamic Shariah, every order is not obligatory. There are orders, for instance, to pray five times a day; and orders to pray 11 rikats after midnight. Every Muslim knows that the first is obligatory, while the second is desired but not obligatory. In the same way, there are some restrictions which are compulsory, while others are not so. A Muslim is forbidden to drink wine, and also he has been forbidden to eat by left hand. The first is حرام (forbidden), while the other is مكروه (Undesirable) but not Haram. So, the restriction against going near a certain tree does not necessarily mean that it was Haram to go near it.
Thirdly, the place where man is to be tested by the rules of Shariah is this world, not the paradise. Man is sent here so that his worth may be tested. After that test, the successful person will be sent to paradise to enjoy the Grace of Allah. There will be no test there. Examination is here in this world. Answers are to be written here, not in the paradise. Paradise is the prize of success in the examination.
Therefore, any order given in heaven, must be, by nature of the place, of an advisory nature, not an obligatory command. If we go against the advice of a wise person, we may put ourselves into inconvenience and trouble; but surely it cannot be said that we committed a crime. The result will be a loss for us and that's all.

Verse of Qur’an About Adam
Now let us examine some verses of Qur'an about Adam one by one:
(1) "We said: O Adam! Dwell thou and thy wife in the Garden; and eat of the bountiful things therein as ye will, but approach not this tree, or ye run into harm.”(Sura al-Baqarah, 2:35, al-A’far, 7:19).
The word used is ظالمين‘Dhalemeen’, which is derived from ظلم ‘Dhulm’. Dhulm means:
1. to misplace a thing;
2, to come to harm;
3, oppress;
4 to make haste before time.
Which of these meanings is suitable here? Here are a few sentences: Cow eats; tiger eats; dog eats; man eats- But we never think that the cow is eating a zebra and the tiger is eating grass, or that the dog is eating the cakes and the man is eating the dog's food.
Though the verb 'Eating' is the same everywhere; but we supply its meaning according to the subject of our conversation. Likewise, as the word 'Dhulm' has at least 4 meanings, when it is used for a prophet and khalifa of Allah (who we have reason to believe must be sinless) we can apply only those meanings which do not imply sin.
If we take the meaning of 'coming to harm', it is clear that God told Adam that if he went near that tree, he would put himself in troubles and hardship; but corning to harm is not a sin or crime. We may apply, with equal force, the meaning of 'making haste before time.' In that case it will mean that though Adam was destined to ultimately come on earth, but if he went near that tree, he would be sent there earlier and with much haste, it must be recognised that we have not invented any new meanings, but, at the same time, we have not declared Adam to be a sinner.
One more note of explanation. 'Sin' is not the actual meaning of ‘Dhulm.’ It is just because a sinner misplaces his faculties and misuses the power given him by God that he is called, metaphorically, ‘Dhalim’
(2) "Then did Satan make them slip from that (Garden) and get them out from the state (of felicity) in which they had been."( Sura al-Baqarah, 2:36)
The word used is ‘Azalla’ (أزَلَّ) which literally means 'made (someone) slip.' It is used, metaphorically, for mistake and sin. But the metaphorical meaning is not the real meaning. Therefore, we may safely translate it that as Adam and Hawwa were sent from a high place to the Earth, as the word in the next sentence 'lhbitu' (go down) indicates, the word slip is more appropriate in this context
(3) “We said: Get ye down, all (ye people), with enmity between yourself, on Earth will be your dwelling place and your means of livelihood — for a time. Then learnt Adam from his Lord some Words, and his Lord turned towards him.”(Sura al-Baqarah, 2:36-37)
Here the word used is (تاب عليه‘taba alaihe’. This verb is from (توبة) Tauba’, which is used for ‘repentance.’ Thus the word, as is generally used, suggests some mistake. But the literal meaning of Tauba is 'to return.' Using in metaphorical sense, it will mean 'God accepted Adam's repentance'; but in literal way, it will mean 'God turned towards Adam (with His Grace and Mercy).' And why should not we translate it in this way? After all, actual meaning must have precedence over the metaphorical one.
(4) The arguments about above verses become clear when we see the following verses in Sura XX:
“Then we said: O Adam! Verily this is an enemy to thee and thy wife- So let him not get you both out of the Garden, so that thou art landed in misery. There is therein for thee not to go hungry nor to go naked, nor to suffer from thirst, nor from the sun's heat.”(Sura Ta Ha, 20:117-119).
The word used is (تشقى) 'Tashqua.' In Muslim books of sermons, it is used for Disgrace in the eyes of Allah. Thus many people are tempted to connect it with the so-called sin of Adam and his coming into disgrace. Actually, this word means 'misery'. The spiritual meaning is just an allegory.
That is why Abdullah Yusufali translated it in these words: 'so that thou art landed into misery'. Strong proof of it is just in the following sentences where the food and shelter etc. have been mentioned- It is clear that God meant that if Adam did not remain on guard and allowed Satan to tempt him, Satan would put him out from heaven and he would remain in misery, because upon Earth he would not get his needs provided for.
This list of the felicities of the Garden supports our view that the word ‘Dhulm’ used in previous ayat also means to come to harm, not sin.
(5) “Thus did Adam disobey his Lord, and allow himself to be seduced.”
The words used are (عصى)‘asa’ and (غوى)‘Ghawa’. The-literal meaning of ‘asa’ is 'disobeyed' and the literal meanings of (غوى) 'Ghawa' are 1) Lost the way, 2) became disappointed, 3) died.
Disobedience is not necessarily sin. Because if the order was of an advisory nature, its disobedience would not amount to sin-And I have explained earlier that the orders in the Paradise cannot be deemed as Wajib or Haram.
Now we may translate it safely: 'Adam did not follow the advice of his Lord, and became disappointed, disenchanted.

Other Prophets
Prophet Musa (Moses) is recorded in Qur'an as saying: (When Pharaohh rebuked him that he (Moses) had killed one Egyptian, and that he was an ungrateful man) "I did it then when I was unaware." (XXVI:20). Hence the word used is ضالين (Dhalleen), which is derived from ضل (Dhalla). It means: i) To forget the way; 2) To wander and wonder; 3) To disappear in search of a thing, whether that thing be good or bad; 4) To go astray from truth; 5) To be destroyed.
Prophet Musa (a.s.) had not killed the Egyptian intentionally. He wanted to prevent him from beating an Israelite. Prophet Musa (a.s.) hit him with his fist and he succumbed and died.
When Prophet Musa was sent by God to Pharaoh, Pharaoh reminded Musa of that action. Even by the standard of the worldly government it was not a murder. Still Prophet Musa used the word Dhall (ضلّ) for himself. There is no need to suppose that he meant that at that time he was 'On wrong path.' He merely says that he did not know his own strength before that time and was unaware that his one slap would kill that man.
There are three or four more references of this kind about other prophets. As I have given above all the meanings of the words used in those verses you can apply your own judgement when searching for their meanings.

Tark-ul-Awla
If this was not a sin, as I have explained so far, then why God has used such strong words? There is a line of poetry in Urdu; which means: "Those whose place is higher, their difficulty is greater." We, the ordinary men, may disregard the advice (not the order) of God several times in a day and it may not be counted against us. Why?
Because the moral or religious standard expected from us is not very exacting. But a prophet is the Vicegerent of God, he is nearer to God, he receives revelations and gets inspirations from God. God wants him to be the model of perfection. Then if he does act in such a way which, though not a sin, is not in keeping with the high standard set for them, God uses the same words which are, ordinarily, used for common people who would have committed a sin. It is beneath their high prestige to ignore even a simple indication of the pleasure of God. It is not sin; but they should not do such acts which may, even apparently, resemble a sin.
And if they do such acts, it is called 'Tark-ul awla' (To leave the better thing). They have left the better way though have not gone astray. This word sums up the whole position. And these hard words serve another purpose also: that is to impress the minds of the believers that when a prophet is put to task in such a way for such a minor act which was not even a sin, how much should we strive to refrain from sins and mistakes. Anybody who will read Qur'an with open eyes will see that those narratives have always been used as examples in a sermon.

Ismat of the Holy Prophet
Now we come to some verses which concern our Holy Prophet in particular:
1st Verse: ''Verily God intends but to keep off all abomination from you, ye Members of the Family, and purify you a thorough purification." (Qur'an, 33:33).
This verse clearly shows that 'Family of the Prophet' was made clean and spotless by Allah Himself.
2nd Verse: "So take what the Apostle gives to you, and refrain from what he forbids you." (Qur'an, 59:7).
It means that the permission or forbiddance by the Holy Prophet was always liked by God. Can anybody be so sure about the commands of a man who is not infallible?
3rd Verse: "Say: If you do love God, follow me: God will love you and forgive you your sins" (Qur'an, Sura Ali-Imran, 3:31).
Here the love of God is made dependent upon following the Prophet of Islam. Both sides of love are included ;n it. If you are loving God follow the Prophet; when you will follow the Prophet, God will love you. Was this imaginable if the Prophet were not free from every type of blemish?
4th Verse: Not only his actions, but even his words were the commands of God. God says in Qur'an: "Nor does he say (aught) of (his own) desire. It is but the inspiration sent to him." (Sura an-Naj, 53:3-4).
Here we are reminded of the prophecy of the Prophet Jesus Christ, recorded in John, 16:7-13: “It is expedient for you that I go away: for if I go not away the Comforter will not come unto you...... for he shall not speak of himself, but whatsoever he shall hear, that shall he speak......."
5th Verse: There are several verses in which the following words have been used for the Holy Prophet: "An Apostle from among themselves...... to purify them and to instruct them in the Book and to teach them Wisdom." (Sura al-Jumu’a, 62:2).
How could a prophet purify others from sins and blemishes if he himself was not pure? How could a man teach others wisdom if he had no wisdom to distinguish right from wrong, or, worse still, no will power to resist from wrong when he knew it.
The Prophet was to teach people the Book of God; It means that he knew the commandments of God. He was to purify them and teach them wisdom: it means that he had wisdom and purity himself. Can a man be called wise if he knew what was not liked by Allah and still committed it?
6th Verse: The witness to the perfection of his character is found in the Qur'an where it says: "Verily; thou art on great character." (Sura al-Qalam, 68:4)
A man committing mistakes cannot deserve such compliments. These and many other verses of Qur'an clearly show that the Holy Prophet of Islam was absolutely sinless and Masum.

Some Misinterpreted Verses
There are some verses in Qur'an about our Prophet which have been grossly misinterpreted by Muslims and non-Muslims alike.
1st Verse: There are three verses in Sura 93, which say, "Did He not find thee an orphan and give shelter (and care)? And He found thee lost and He guided. And He found thee in need and made independent. (Sura adh-Dhuha, 93:6-8).
Our Prophet was an orphan; his father died before he was born, his mother died when he was 6 years old; his grandfather died when he was 8 years of age He finally came under the loving care of his uncle Abu-Talib. In all the places mentioned above, he found more love and care showered upon him than could be expected from a father.
In ayat no. 6 Allah reminds the Prophet about His Grace which was bestowed upon him in the house of Abdul Muttalib and Abu Talib, the Holy Prophet was a poor man; but after his marriage with Khadija (one of the four chosen women in the world, according to the traditions of the Prophet), the pure and sincere love of Khadija not only raised him above want, but made him independent of worldly needs in his later life. In verse no. 8 God reminds the Prophet about His Grace in the person of Khadija.
In between these two Graces, he reminds the Prophet that he was lost and Allah gave guidance. The word used is “Dhaall” (ضلّ) and it has been mentioned earlier that 'Dhaall' means among other things, 'to be lost,' 'to wander in search of anything.' Thus it may be translated as, "And He found thee wandering in search of truth and He gave thee guidance” There is no implication whatever of sin or error on his part.
But even if we insist upon the meaning of "being lost," it does not mean that the Holy Prophet had gone astray. Ibn-e-Abbas and Imam Raza (a.s.) both have interpreted this verse as meaning that "you were lost among your people (i.e., the Meccans did not know your actual position, your spiritual excellence), so He guided" (them to know you).
This meaning is in conformity not only with the reason and other verses mentioned above, but also conforms with the second verse of 53rd Sura where it declares: "Erred not your companion (i.e., the Holy Prophet) nor was he led astray”
2nd Verse: There are some verses ordering the Prophet to ask forgiveness: "And ask forgiveness for thy fault." (40:55).
"That God may forgive thee thy faults of the past and those to follow" (48:2).
Its true significance will be understood when we remember that the Exalted Excellence of Allah, according to Islam, is beyond human imagination. Nobody, however perfect, can think, in Islamic world, that he had perfected his love and obedience to God. Thus, our Holy Prophet used to pray whole nights till his feet became swollen by remaining standing in the prayers, and God in His Love and Kindness told him: "We have not sent down the Qur'an to thee so that thou should put thee in trouble." (20:2).
Still, he used to say: "I did not worship Thee as much as befitting Thy Excellence." And: "I did not worship Thee which would have been befitting Thy Perfectness." There is no question of sinning here but there is a feeling, which comes from true Love of Allah, that one is not doing enough for God, one is not doing what may be deemed befitting to the highest position of the Almighty Creator. It was because of this feeling that the Holy Prophet (and Imams) used to "repent" before God for their "shortcomings." But, in fact, there was no short-coming at all.
3rd Verse: Third type of the verses are those in which the Qur'an uses the style of "I am telling thee, but I want the neighbour to hear." It means that to emphasize a point, God addresses a rebuke directly to the Holy Prophet, while in fact the topic does not concern him at all. In such verses, though the pronoun used is the second person singular, but the topic is intended for the Muslims (or the whole mankind) in general.
One example is: "If though committed polytheism surely thy good deeds would be forfeited." (39:66).
It is a warning to the mankind in general; not a rebuke to the Holy Prophet, who had never committed polytheism. Such verses cannot be used to say that the Holy Prophet had, God forbid, committed any sin. These verses were addressed to him; but were actually meant for his Ummat (people).