Question: According to Qur’an's point of view one of the effects of Prayers is that it keeps one away from sins and unlawful things. Because Qur’an says: And keep up prayer; surely prayer keeps (one) away from indecency and evil…[1]
Then also we see that many people who pray commit sins and evil deeds and there is no effect of prayers on their words and deeds. Then what is the meaning of the above-mentioned ayah?
Answer: First of all one should remember that for a person to remain away from sins, the primary thing is remembrance of Allah and Faith. One, who forgets Allah, doesn't think about his actions and character and to fulfill his desire never thinks of any limit. The opposite of this is that the remembrance of Allah keeps us away from sensual desires. The remembrance of that God who is All-aware of our small and big deeds and whatever we think, He is all knowing.
To control unlawful desires, the natural and straight way is to remember God, to know the status of godly people, to remember the punishment meant for sinners and to remember the rewards for those who follow the divine commandments.
The holy Qur’an describes the godly people in this way: Men whom neither merchandise nor selling diverts from the remembrance of Allah and the keeping up of prayer and the giving of poor-rate…[2]
Amir al-Mu’mineen Ali (a.s.) gave special importance to the remembrance of Allah and says as follows: “Allah the most High has created lights in the hearts of his servants, due to that shining their ears start listening and eyes start seeing and they leave enmity and mischievousness and obey His commands.”[3]
Imam Ali (a.s.) in these sentences said that 'forgetfulness' is the disease of the heart and deafness and blindness, which make the human beings rebellious, quarrelsome and inimical from the true and right path. In comparison to this your property is the remembrance of Allah, through which your heart listens and enlightens, which bring the sensual desires under the control.
On the basis of whatever is said above, one who is unmindful about the rewards and punishment of Allah is like a blind and deaf, who rides an untrained horse. certainly the horse will hit the stones and fall into a pit, but those people who are awakened and their hearts remember the Lord; they closely observe the results of their deeds. They listen to the truth with great attention and keep their sensual desires under control by remembering the Almighty.
Imam al-Baqir (a.s.) addresses one of his friends in this way: “Always remember Allah, because His remembrance is a hindrance between a person and unlawful deed.”[4]
Asbagh ibn Nubata says the Amir al-Mu’mineen (a.s.) said: “Remember Allah on two places, one at the time of distress and when you are about to commit a small sin, and to remember Allah at the time of sin is more important because His remembrance is a hindrance between the person and the sin.”
It is apparent that there are different stages of remembering Allah, and they are never similar. Sometimes he reaches the stage that insures a person against all sins and due to this his involvement in sin naturally becomes impossible. These are people who remember Allah with a peaceful heart, awakened heart, enlightened eyes, hearing ears and obedient soul.
Most of the people remember God in a middle stage. For example there are some people who commit some sins, but in any way they do not shed the blood of others, or usurp the property of orphans, or defame others because they know the punishment for these sins is very severe and their inner soul stops them from these sins.
Sometimes the remembrance of Allah is so less and hazy that it becomes apparent only in leaving of sins. Many times the stronger factors, makes its basis effect less. But if the remembrance of Allah is practiced even if weak and defective, it can protect a person from many sins.

Prayers is the source for the remembrance of Allah
One of the secrets of Prayers is that it inculcates the remembrance of Allah in man and Qur’an itself says that it is one of the purposes of Prayers: Therefore serve Me and keep up prayer for My remembrance…[5]
It is apparent that Prayers is worship and it should be performed with the purpose of nearness to Allah, which is why it is naturally the cause for the remembrance of Allah.
The person who is praying expresses those sentences by his tongue which makes him attentive towards Allah and it is the source of His remembrance. For example in the Surah Hamd, which we recite in the Prayers, we praise Allah and His qualities and after this in his holy realm mention the different servants and it also same for other recitations in the Prayers.
The effect of Prayers on our soul is that it makes our spirit stronger and makes us remember Allah, and this remembrance as we have told earlier has different stages. Those people who do not refrain from many sins and are careless and due to sins they do not remember Allah.
In other words, the saying of Qur’an that “Prayers protects the person from sin” doesn't mean that the person who prays Prayers becomes immune from sins, but it means that it is the cause of remembrance of Allah and it also diverts a person attention towards the Lord and the natural effect of this type of remembrance creates a spirit in the worship and refrains him from sins. But it is also possible that if a person is less attentive towards Allah, the other factors may remove the spiritual effect.
The conclusion of this discussion is that if the Prayer is prayed in its proper way, it will have an effect on the person to make him resist sins. Sometimes this effect is strong and sometimes it is weak and there is variation due to the difference in sins and difference in Prayers. When the Prayers are prayed with humility and perfection, his training and restraint from sins is stronger.
Prayers of those who pray practically stops them from sins and along with this help in refraining from other sins because the worshipper has no option, that for making his Prayers right and acceptable, he should refrain from many sins. For example one of the conditions of Prayers is those things, which are necessary in Prayers should be lawful. This makes the worshipper leave many things, because it is very difficult that a person who prays makes sure that all the things required for Prayers be lawful and in other things is not worried about lawfulness.
From whatever is said above it is proved that those who do not pray, as a result of non performance of Prayers they also fail to perform other worship acts like fasting, Hajj, Khums and Zakat. They do not differentiate between Halaal and Haraam, clean and unclean. While those who at least pray, refrain from some sins and those who perform this worship in a better and perfect way refrain from most of the sins or all the sins.
In brief Prayers have a training effect on the person who prays, whether this effect be comprehensive or weak, it depends on the performance of Prayers and upon its manner and spirit.

Notes:
[1] Surah Ankabut 29:45
[2] Surah Nur 24:37
[3] Nahjul Balagha, Sermon 222
[4] Biharul Anwar, Vol. 1, p. 486
[5] Surah Taha 20:14