“The difference between the Scholar and the dâ’î is clear. The dâ’î is one who strives to convey the message of the Sharî’ah to the servants of Allâh. He calls them to it, sometimes by means of targhîb and tarhîb (persuasion and deterring).
The Scholar is one to whom Allâh has given knowledge and who may or may not be a dâ’î. However, if the Scholar is not a dâ’î., then he is extremely deficient in his knowledge and is not a complete inheritor of the Messenger of Allâh sallallâhu ’alayhi wa sallam. This is because the Prophets - may Allâh’s peace be upon them all - did not bequeath the dirham or dînâr as inheritance, but they bequeathed knowledge - as the Prophet sallallâhu ’alayhi wa sallam said: “Indeed, the Scholars are the inheritors of the Prophets and indeed the Prophets do not leave behind them the dînâr or the dirham as inheritance, they leave only knowledge behind as inheritance. So whosoever acquires it, acquires a huge fortune.”  Consequently. whosoever acquires knowledge and calls to Allâh. then such a person has truly inherited from the inheritance of the Prophets - in proportion to what he establishes and implements of their prescribed laws.
As for the saying of some of the people that it is permissible to become a dâ’î without knowledge, then if they mean a dâ’î without a huge amount of knowledge, able to give fatâwâ (legal verdicts and judgements), explain and deduce issues from their proofs - then it is possible to accept this saying. However, if they mean a dâ’î not having knowledge what to do da’wah with, nor having knowledge what to do daw’ah too - 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 good achieved - as is witnessed!”
(1) Hasan: Related by Abû Dâwûd (no.3641), and Ibn Majah (no.223), from Abûd-Darda radiallâhu ’anhu. It was authenticated by Shaykh al-Albânî in his checking to Sharhus-Sunnah (1/276).