It is essential for the person who calls people to Allaah, and enjoins what is good and forbids what is evil to have knowledge because Allaah says (interpretation of the meaning):

“Say (O Muhammad صلى الله عليه وسلم): This is my way; I invite unto Allaah (i.e. to the Oneness of Allaah — Islamic Monotheism) with sure knowledge, I and whosoever follows me (also must invite others to Allaah, i.e. to the Oneness of Allaah — Islamic Monotheism with sure knowledge)”

[Yoosuf 12:108].

Knowledge is what Allaah says in His Holy Book, or what His Messenger (blessings and peace of Allaah be upon him) says in his saheeh Sunnah. So one should learn the holy Qur'aan and the Sunnah, to find out what Allaah has enjoined and what Allaah has forbidden, and to know the way in which the Messenger (blessings and peace of Allaah be upon him) called people to Allaah and denounced evil, and the way in which his Companions (may Allaah be pleased with them) called people. He can gain insight into that by studying the books of hadeeth, and paying attention to the holy Qur'aan, and studying the words of the scholars on this topic, because they discussed it in great depth and explained what is obligatory. 

The one who wants to set himself up as a caller must pay attention to this matter, so that he will have proper insight into the Book of Allaah and the Sunnah of His Messenger (blessings and peace of Allaah be upon him), so that he will put things in their right places: he will call people to good when it is appropriate, and enjoin what is good when it is appropriate, with insight and knowledge, so that he will not denounce evil in a way that is more evil than it, and so that he will not enjoin what is good in a manner that will result in an evil worse than not doing the good thing to which he is calling people. 

The point is that it is essential for him to have knowledge so that he will do things properly. End quote.