the necessity of excercising an effective knowledge in bidding to good and forbidding evil
What we mean here is that amr bi 'l-ma'ruf wa nahy `ani 'l munkar (bidding to good and forbidding evil) has as its purpose that ma'ruf (that which is good, beneficial) should be propagated and munkar (that which is detestable, atrocious) be effaced. Thus there must be bidding to good and forbidding of evil in a place where there is the possibility of the desired effect coming about. If we know that there will definitely be no effect, how can it continue to be an obligation?
Moreover, the purpose in the legal (in Islamic terms) basis of this activity is that what is of advantage should be carried out. Thus it must obviously take place in a situation where there will not result any greater disadvantage. The requisite for these two conditions, then, is a thorough understanding of how to act correctly. A man who is lacking in this knowledge cannot foresee whether the desired result of this action will follow or not, or whether some greater evil will be produced or not. This is why the corruption resulting from ignorant inciting to good will be greater than its benefit, just as has been related in hadith.
In the context of other duties, it has not been laid down as a condition that there must exist the possibility of their producing a useful result, and that if there is that possibility they become obligatory, otherwise not. Although something useful and of benefit manifests itself in every duty, the recognition of that benefit is not the responsibility of people. It has not been said about ~prayer~, for example, that if you see that it is useful then pray, and if you do not, then do not pray. Neither is it said about fasting that if it contains the possibility of producing something beneficial then fast, and if it does not have that posssibility then do not fast (only in fasting it is said that if you see there is harm in it, then do not fast), and likewise in ~hajj~ or zakat or ~jihad~ there is no such restriction. But such a restriction does exist in the matter of bidding to good and forbidding evil, that one must look to see what kind of result, and what kind of reaction will be produced, and whether the action is in the interests of Islam and Muslims or not. That means that the discernment of the benefit is the responsibility of the very people who carry out this duty.
Everyone has a share in this duty, but it is necessary that he introduces reason, intelligence, knowledge of how to act correctly and attention to its benefit, and these latter things are not merely a matter of religious obligation.
This condition, that it is necessary to exercise a knowledge of effective action in bidding to good and forbidding evil, is unanimously agreed upon by all the sects of Islam except the ~Khawarij~. Because of their particular inflexibility, rigidity and fanaticism, they said that bidding to good and forbidding evil is an absolute religious obligation; it has no condition of the possibility of a useful result or the absence of any corrupting influence; one must not sit down and think about it. It was in accordance with this belief that they rose up and terrorised the lands knowing that they would be killed and their blood would be wasted, and knowing that no useful result would come out of their uprising.
Sources :
attraction and repulsion of Imam Ali p.b.u.h- pages: 114to115