Is Islam actually a Christian heresy? Was it actually not a “new religion,” but rather, a perversion of orthodox Christian teaching?
1,400 years after Mohammed supposedly received his revelation from the archangel Gabriel, it's difficult to determine. But nevertheless, it's interesting to note that this charge has been made by some influential figures throughout history.
In his work Fountain of Knowledge, one of the most renowned Fathers of the Church—St. John of Damascus (675-749)—claimed that Mohammed was in part inspired by a monk who subscribed to the Christian heresy of Arianism. As a post on the Christian site Patheos explains:
“St. John had grown up in the Ummayad Arab court of Damascus, where his father was chancellor, and he was an intimate boyhood friend of the future Caliph al-Yazid; the two boys’ drinking bouts in the streets of Damascus were the subject of much horrified gossip in the streets of the new Islamic capital. Later, in his old age, John took the habit at the desert monastery of Mar Saba where he began work on his great masterpiece, a refutation of heresies entitled the Fount of Knowledge. The book contains an extremely precise and detailed critique of Islam, the first ever written by a Christian, which, intriguingly, John regarded as a form of Christian heresy related to Arianism: after all Arianism, like Islam, denied the divinity of Christ. Although he lived at the very hub of the early Islamic world, it never seems to have occurred to him that Islam might be a separate religion.”
St. Thomas Aquinas (1225-1274)—considered one of the greatest minds in the history of Western civilization—thought the same, as did much of the Christian world in the Middle Ages.
And in his 1938 book The Great Heresies, Hilaire Belloc, an influential Catholic author of the 20th century, reiterated the charge:
“Mohammedanism was a heresy: that is the essential point to grasp before going any further. It began as a heresy, not as a new religion. It was not a pagan contrast with the Church; it was not an alien enemy. It was a perversion of Christian doctrine. Its vitality and endurance soon gave it the appearance of a new religion, but those who were contemporary with its rise saw it for what it was—not a denial, but an adaptation and a misuse, of the Christian thing. It differed from most (not from all) heresies in this, that it did not arise within the bounds of the Christian Church. The chief heresiarch, Mohammed himself, was not, like most heresiarchs, a man of Catholic birth and doctrine to begin with. He sprang from pagans. But that which he taught was in the main Catholic doctrine, oversimplified. It was the great Catholic world—on the frontiers of which he lived, whose influence was all around him and whose territories he had known by travel—which inspired his convictions.”