Was Islam a Heresy? John of Damascus and the First Christian Response
Christianity and Islam both claim to be revelations of the one true God, but at their core, they proclaim conflicting truths that cannot be reconciled. John of Damascus, an early Christian theologian, was among the first to recognize and articulate these doctrinal contradictions. He called Islam “the heresy of the Ishmaelites,” not out of hatred, but because it deviated sharply from historic Christian truth.
The following are from On Heresies, in which John of Damascus (675–749 AD) identifies how Mohamed (c. 570–632 AD) invented Islam.
You can read more about this in Daniel J. Janosik’s book.
“There is also the superstition of the Ishmaelites (John’s term for Muslims, emphasis mine) which to this day prevails and keeps people in error…”
— On Heresies, §101
“From that time to the present, a false prophet named Mohammed has appeared in their midst.”
“He says that Christ is the Word of God and His Spirit, but a creature and a servant, and that He was born without seed of Mary the sister of Moses and Aaron.”
“They say that He [Jesus] was a prophet and a servant of God, and that He ascended into heaven without having been crucified…”
(This references Surah 4:157, denying the crucifixion.)
“This Mohammed, as it has been said, wrote many ridiculous books, to each one of which he set a title…” (Referring to the Qur’an’s surahs)
“He says that the Jews wanted to crucify Him, but they were unable and crucified their own son instead.”
“This is what he says, and this is what he wrote in his book, which he calls the Qur’an.”
The following are key doctrinal distinctives between Christianity and Islam
The Trinity is at the heart of Christianity: one God in three persons—Father, Son, and Holy Spirit. Islam rejects this as blasphemy. The Qur’an declares, “Say not ‘Three’: desist—it is better for you. Allah is only one God. Glory be to Him—that He should have a son!”
— Surah 4:171 God, in Islamic theology, is one in essence and person, a concept that directly opposes the Christian confession.
Another sharp divergence lies in the person of Jesus Christ. Christians worship Jesus as the eternal Son of God, fully divine and fully human, crucified for our sins and risen for our salvation (John 1:1, Romans 4:25). Islam honors Jesus (“Isa”) as a prophet, but denies his divinity: “The Messiah, son of Mary, was no more than a messenger; many were the messengers that passed away before him…”
— Surah 5:75.
Islam also denies the crucifixion and, therefore, rejects his role as Savior: “And [for] their saying, ‘We killed the Messiah, Jesus, the son of Mary, the messenger of Allah.’ But they did not kill him, nor did they crucify him; it was only made to appear so to them…”
— Surah 4:157
Finally, the doctrine of grace in Christianity is foreign to Islamic theology. In Christianity, salvation is a gift received by faith in Christ (Ephesians 2:8–9). In Islam, salvation depends on balancing good and bad deeds (Surah 23:102–103) and operating within the Five Pillars of Islam.
These differences are not minor. They go to the root of who God is, who Jesus is, and how salvation is received. Respectful dialogue is possible, but honest dialogue requires clarity: both religions cannot be true at the same time.