Q.1 Is da’wah (calling) to Allaah obligatory upon every Muslim man and woman, or is it to be left for the Scholars and the students of knowledge only? Is it permissible for the layperson to do da’wah to Allaah?
A.1 Shaykh ibn al-’Uthaymîn – rahimallah – replied
“When a person has knowledge and insight into that which he is calling to, then there is no difference between the one who has a great amount of knowledge, or a student of knowledge who has recently started in pursuit of knowledge or a lay person – as long as he has certain knowledge of the issue at hand. The Prophet sallallâhu ’alayhi wa sallam said: “Convey from me, even if it is one verse.”  So it is not a condition upon the dâ’î (the one calling) to attain a great amount of knowledge, but the condition is that one must have knowledge of what one is calling to. If this calling is established upon ignorance and built upon emotion and passion, then it is not permissible.
Thus, we see that some of the brothers who call to Allaah, they do not have except a little knowledge. We see them, due to their strong emotions, prohibiting that which Allaah has not prohibited, whilst making obligatory that which Allaah has not made obligatory upon His worshippers. This is a very dangerous matter, since permitting what Allaah has made harâm (unlawful) is like prohibiting what Allaah has made halâl (lawful). So when they begin prohibiting people for making a particular matter then others will rebuke them for making it harâm. Allaah – the Most High – says: “And do not say, concerning that which your tongues falsely put forward, ‘This is lawful and this is forbidden,’ so as to invent lies against Allaah. Indeed, those who invent lies against Allaah will never prosper.” 
As for the layperson, then he must not call to Allaah if he does not have knowledge. Rather, it is essential to have knowledge in accordance with the saying of Allaah – the Most High -: “Say: This is my path. I call to Allaah upon sure knowledge.”  So it is a must to call to Allaah upon knowledge. However, if a matter is clearly known to be evil or good, then one can command it – if it is good, or forbid it – if it is evil. So the callers to Allaah must start with knowledge. Whosoever calls to Allaah without knowledge, then such a person will cause greater harm than good – as is evident. So it is obligatory for a person to first acquire knowledge, then to do da’wah. As for the clear evils and that which is clearly good, then the good is enjoined and the evil prohibited.” 4
1. Related by al-Bukhârî (no.3461) from ’Abdullâh ibn ’Amr radiallâhu ’anhu.
2. Sûrah Nahl 16:116-117
3. Sûrah Yûsuf 12:108
4 As-Sahwatul-Islaamiyyah (pp.75-76) of Shaykh lbn al-’Uthaymîn.