They rectified heretical innovation

Now God sent to His servants by the mouth of His messenger, in the Koran and traditions, a creed which is the truth and whose contents are the basis of man’s welfare in both religious and secular affairs. But Satan too sent, in the suggestion of heretics, things contrary to orthodoxy; men tended to accept his suggestions and almost corrupted the true creed for its adherents. So God brought into being the class of theologians, and moved them to support traditional orthodoxy with the weapon of systematic argument, by laying bare the confused doctrines invented by the heretics, at variance with traditional orthodoxy. This is the origin of theology and theologians. In due course a group of theologians performed the task to which God invited them; they successfully preserved orthodoxy, defended the creed received from the prophetic source, and rectified heretical innovation.

— from Deliverance from Error, by Al-Ghazzali

Al-Ghazzali (c. 1058–1111) is also known in Islam as Abu Hammid Muhammad ibn Muhammad al-Ghazzali, a Muslim theologian often considered the most influential Muslim after Muhammad himself. He is simultaneously known for bringing the philosophical investigation of Islam through the Greek philosophers, especially Aristotle and Plato, to an effective end while also unifying many elements of the fundamentalist orthodox and mystical Sufi elements of Islam, thus strengthening the faith’s unity.