Medieval Christian thinkers could not conceive of non-Scriptural sources of truth

Evidence

Scripture was the framework of all mediaeval thought; for no Latin writer was it ever possible to think himself into a position outside and independent of Scripture. … It was inconceivable that the Qur’an should be the sole source, or sole reliable source, of knowledge of the prophets, or that the prophecies should not have been concerned exclusively with the Kingdom of the Messias. … The Old Testament would always seem to be the primary source of such knowledge, and therefore in practice the Old Testament was used to refute the Qur’an. — Norman Daniel, Islam and the West, 1403