Shaikhul Islaam Ibn Taymiyyah (rahimahullaah) said:
One must have knowledge of the Ma’roof and the Munkar [i.e. knowledge of the good affair he (or she) wants to command a person and the evil affair he wants to forbid], and (be able) to distinguish between them. And one must have knowledge of the situation of the one he (or she) commands with good and forbids from evil.
[Al-Amr Bil Ma’roof Wan-Nahy Anil Munkar’ page 55]