The Messenger of Allah (S) has said, “O' mankind! Do not convey wisdom to those people who are not worthy (of receiving such teachings), as you would have done oppression to the wisdom; and do not deprive those people who are worthy of receiving such teachings, as then you would have done oppression to them.
Do not seek revenge from the person who has oppressed you for if you do, you would make your worth and value null and void; and do not perform (good) acts for other people to see, for through this you would eliminate (the good) of such acts. Do not prevent yourselves from giving charity otherwise you will reduce the good that is to come to you.
O' Mankind! Verily things are of three types: (1) some of which whose correctness is apparent to you so then follow these (and perform them); (2) actions whose incorrectness is apparent to you so then avoid performing these acts; (3) actions whose performance is of doubt, so then refer the ruling on these to Allah…”[1]
This tradition consists of five sections. In the first section, the Prophet of Islam (S) has given us five commandments:
1. We must not convey wisdom and knowledge to those who are not worthy of such teachings, for then we will do oppression to knowledge.
2. We must not deprive those people who are worthy of gaining knowledge from our knowledge since if we do this, we will commit oppression to those people.
From the wordings of this part of the tradition we can deduce that there are specific conditions which must be met for a student of knowledge - the most important is that one must have the (spiritual) ability to accept such knowledge and if this is not within a person, then one is not worthy of acquiring knowledge - that knowledge whose acquisition has been promised a great reward.
The Prophet (S) has stated that, “If there is a person who is not worthy of gaining a (particular) knowledge, then you must not teach it to him since when knowledge falls into his hands, he will use it in the wrong way and will lead the world to destruction. A person who is ignorant and lacks knowledge neither has the power to destroy a land, nor to make it habitable.”
In the present time, those people who are leading the world into corruption are the ones who are at the head of the colonializing governments of the world through the guidance of their self-proclaimed “scholars” and “intellectuals”.
The Qur'an contains many different expressions such as:

هُدىً لِّمُتَّقِينَ

”A guidance for those who have consciousness (of Allah).”[2]

إِنَّ فِـي ذٌلِكَ لَأَيَاتٍ لِّقَومٍ يَسْمَعُونَ

“Surely in that (the day and the night) are signs for a people who listen.”[3]
It is phrases such as these which clearly show us that knowledge without spiritual purification bears absolutely no (positive) results.
Thus, it is clear that true guidance is specifically for those people who have already laid the groundwork (to accept knowledge and further guidance). Based on this, we see that in the past, the true scholars did not give permission to just any student to come and sit in their classes - rather, they tested them in relation to their ethical morals and from this, they would see what level of piety the students were at.
However this does not mean that one has the right to hoard his knowledge! Rather, one must only teach such knowledge to those who are worthy of receiving it and it is through the knowledge that one possesses that he must try to cure the ailments of the people. It does not make a difference if the ailments are in relation to their material life or their spiritual life, although we believe that spiritual ailments and diseases are much worse than the material or physical sicknesses. We believe this since we accept the fact that Allah (SwT) will hold a person responsible for his spiritual illnesses (and not his physical sicknesses), just as it has been mentioned in the traditions that:

مَا أَخَذَ اللٌّهُ عَلى أَهْلِ الْجَهْلِ أَنْ يَتَعَلَّمُوا، حَتّى أَخَذَ عَلى أَهْلِ الْعِلْمِ أَنْ يُعَلِّمُوا

“Allah has not taken a covenant from the ignorant people to learn until he took a covenant from the scholars to teach (the ignorant people).”[4]
In the religion of Islam, learning as well as teaching are both obligatory (wajib) and these two acts are not separate from each other since they are the prerequisite and necessity of one another.
3. If an oppressor commits oppression upon you and you take revenge, then your spiritual worth will be removed because you would end up being just like him.
Of course forgiving the person who has oppressed you should only be carried out when you know that he will not take advantage of your forgiveness and that you forgiving him would not have a negative impact on the entire society (by him then committing other acts of aggression on others under the presumption that they too would forgive him).
4. You must perform all your actions sincerely for Allah (SwT) and must not entertain even the slightest amount of showing off in them.
This is a very difficult task to accomplish and as we know, showing off in one's actions is only one of the springs of corruption of one's good deeds. In this regards, there are many other things such as pride, one's own inner passions and desires, etc… that may come into play to corrupt one's good deeds, and spoil the good work that one performs.
For example, sometimes we perform the Salat so that we become pleased with our own selves and we do not think about other people - this in itself is a form of corrupting our good deeds. Or for example we perform the Salat out of habit, or we perform the Salatul Lail so that we can become better than other people or… and it is these and other things that can lead to the corruption of our good deeds.
5. If a person wants something from you, then do not be stingy in relation to it because if you do not give it to the person (yet you are able to) then Allah (SwT) will cut off the bounties that would come to you since:

كَمَالُ الْجُودِ بَذْلُ الْمَوْجُودِ

“The perfection of a person is that he gives what he has.”
If a host does not bring to his guests whatever he has at home (when guests come over to his house), then he has done oppression to them and in similar way, if the guests ask the host for an excess of things, then they would be counted as oppressors!
The second part of this tradition is divided up into three sections:
1. The actions whose correctness is apparent for all to see.
2. The actions in which the misguidedness and corruption is apparent for all to see.
3. The actions which are obscure from the point of view of:
a. The subject matter.
b. The Islamic ruling.
This tradition is in relation to the actions in which the Islamic ruling is one of doubt or obscurity and in some traditions, in place of the phrase:

رُدُّوهُ اِلَى اللٌّهِ…

“Return the (ruling of that particular act) back to Allah.”
it has been mentioned that in those types of actions where we have a doubt as to its ruling, we must exercise precaution, because the questionable actions are the precursor to the forbidden acts.
There are some people who have are habituated in saying:

كُلُّ مَكْرُوهٍ جَايِزٌ

“All the makruh (reprehensible) acts are permitted to be performed.”
To such people who state this, we say to them that yes it is true that we are permitted to act upon an apparent ruling on a particular issue, however when there is clearly a doubt on a particular ruling, then if a person pollutes himself with performing that doubtful action, then slowly the indecency of that particular act with become less in his estimation and thus, he will fall into performing the prohibited!
Allah (SwT) has told us, “Be careful of the evil footsteps of Shaitan” and as we know, one of the most clear examples of following the footsteps of the Shaitan is to perform the acts in which there is a doubt.
Shaitan even misleads the holy, Salatul Lail performing people in a special way. He does not come to them and tell them, “Go out and drink alcohol”. Rather, he starts out by saying, “Salatul Lail is not one of the obligatory Salat, so go ahead and stop performing it.” Once the person leaves this Salat, then slowly, Shaitan would approach him in relation to performing the obligatory Salat right when the time sets in and will say to him, “It is not a condition of the Salat that you perform it right when the time sets in…” and he would continue in this way until the person becomes completely detached from Allah (SwT).
If a person truly wishes to develop a deep spiritual insight and gain spiritual pleasure, then he must keep away from doubtful foods, gatherings, and speech and without doubt, in relation to all of the actions that he performs, the utmost of precaution must be exercised!

Notes:
[1] Bihar al-Anwar, vol. 74, pg. 179
[2] Surat al-Baqarah (2), Verse 2
[3] Surat Yunus (10), Verse 67
[4] Bihar al-Anwar, vol. 2, pg. 80