As for the saying of some of the people that is is permissible to become du’aat (callers) without knowledge, then if they mean a daa’ee without a huge amount of knowledge, able to give fataawaa (religious verdicts), explain and deduce issues from their proofs – then it is possible to accept this saying.
[Q.1]: Is da’wah (calling) to Allaah obligatory upon every Muslim man or woman, or is it to be left for the Scholars and the students of knowledge only? Is it permissible for the lay person to call to Allaah?
[A.2]: 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 (sallallaahu ’alayhi wa sallam) said: “Convey from me, even if it is one aayah.”  So it is not a condition upon the daa’ee (caller) 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 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 haraam (unlawful) is like prohibiting what Allaah has made halaal (lawful). So when they begin prohibiting people from making a certain matter halaal, then others will rebuke them for making it haraam. Allaah the Most High says:
“And do not say, concerning that which your tongues falsely put forward, ‘This is lawful and this is prohibited,’ so as to invent lies against Allaah. Indeed those who invent lies against Allaah will never prosper.” [Sooratun-Nahl 16:116-117]
As for the lay person, then he must not call to Allaah if he does not have knowledge. Rather, it is essential to have knowledge, in accordance to the statement of Allaah the Most High:
“Say: This is my path, I call unto Allaah upon sure knowledge.” [Soorah Yoosuf 12:108]
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 prohibit 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 upon a person to first acquire knowledge, then to give da’wah. As for the clear evils, and that which is clearly good, then the good is enjoined and the evil prohibited. 
[Q.2]: What is the difference between a Scholar and a daa’ee?
[A.2]: The difference between the Scholar and the daa’ee is clear. The daa’ee is the one who strives to convey the message of the Sharee’ah to the servants of Allaah. He calls them, sometimes by means of targheeb and tarheeb (persuation and deterring).
The Scholar is the one to whom Allaah has given knowledge and who may, or may not be a daa’ee. However, if the Scholar is not a daa’ee, then he is extremely deficient in his knowledge, and not a complete inheritor of the Messenger of Allaah sallallaahu ’alayhi wa sallam). This is because the Prophets (’alayhimus-salaatu was-salaam) did not bequeath the dirham nor the deenaar as inheritance, but they bequeathed knowledge, as the Prophet sallallaahu ’alayhi wa sallam) said: “Indeed the Scholars are the inheritors of the Prophets, and indeed the Prophets do not leave behind them the deenaar nor the dirham as inheritance. They leave behind only knowledge as inheritance. So whosoever acquires it aqcuires a huge fortune.”  Consequently, whoever acquires knowledge and calls to Allaah, then such a person has truly inherited from the inheritance of the Prophets – in proportion to what he establishes and implements from their prescribed laws.
As for the saying of some of the people that is is permissible to become du’aat (callers) without knowledge, then if they mean a daa’ee without a huge amount of knowledge, able to give fataawaa (religious verdicts), explain and deduce issues from their proofs – then it is possible to accept this saying. However, if they mean a daa’ee not having knowledge what to give da’wah to, nor having knowledge what to give da’wah with, then there is no doubt that this cannot be. And I warn people from calling to the truth in this manner, since the harm caused is greater than the goodness achieved, as can be seen! 
 Related by al-Bukhaaree (no. 3461) from ’Abdullaah Ibn ’Amr (radiyallaahu ’anhu).
 Taken from as-Sahwatul-Islaamiyyah (p. 75-76) of Shaykh Ibnul-’Uthaymeen, as compiled by ’Alee Aboo Lawz.
 Hasan: Related by Aboo Daawood (no. 3461) and Ibn Maajah (no. 223) from Abud-Dardaa‘ (radiyallaahu ’anhu). It was authenticated by Shaykh al-Albaanee in his checking upon Sharhus-Sunnah (1/276).
 Taken from as-Sahwatul-Islaamiyyah (p. 76-77)