What knowledge is required of one who calls to Allah and enjoins good and forbids evil?

A caller to Allah, an enforcer of good, and a denier of evil must possess knowledge and insight, for Allah, the Almighty, says: (Say, "This is my way: I invite to Allah with insight, I and those who follow me.") And knowledge is gained from the Holy Quran and the authentic Sunnah, and by understanding the method of the Messenger of Allah, peace and blessings be upon him, and his companions in calling to Islam and denouncing evil, and by reviewing books of Hadith and the sayings of scholars. This enables one to put matters in their correct places, so he does not denounce an evil with something worse than it, nor does he enjoin a good that leads to a more dangerous evil.