He had embraced the Motazilite doctrine about free will and predestination, and was in particular shocked at the opinion which had spread among the Moslem doctors that the Koran was the uncreated word of God.
In the Hesychast controversy he took the side of the monks of Athos, but refused to agree to the theory of the uncreated light.
But though he was a student and friend of Ahmad ibn Hanbal he did not go in traditionalism to the length of some, and he defended al-Bukhari when the latter was driven from Nishapur for refusing to admit that the utterance (lafz) of the Koran by man was as uncreated as the Koran itself (see Mahommedan Religion; and Patton's Ahmad ibn Hanbal, 32 sqq.).
The caliph also shared Mamun's intolerance on the doctrinal question of the uncreated Koran.