What is Islam to Christianity?

I write you from Riyadh, Saudi Arabia, the home of Islam. Despite growing up in America as a Catholic, I’ve always loved the Middle East. That’s down to my love for Indiana Jones and traveling through the region as a professional and tourist.

I’ve taught here once and in Abu Dhabi many times. I’ve also traveled through Dubai, Qatar, Turkey, Egypt, Tunisia, and Morocco. The cuisine is delectable, and the people are friendly. Their architecture is unique, and their art sublime.

And no, I don’t hold them responsible for 9/11. There are far more culpable members of the USG from the early 2000s. Did we ever repeal the Patriot Act? Ah. I didn’t think so.

Whenever I come to this part of the world, I wish the same thing: that we could live in peace with each other and not constantly worry about being blown to pieces.

I can’t remember where, but I heard someone say, “Islam is nothing but a Catholic heresy.” I thought the comment interesting and provocative, but didn’t inquire further. I probably thought it best not to add fuel to a possibly incendiary situation.

In the modern, pluralistic West, where we must coddle every creed and treat every belief system as equally valid, the mere suggestion that Islam might be a heresy sounds like cultural suicide. Try floating that line at a cocktail party in New York, London, or Brussels and watch the eyebrows arch and the exits clear.

But what if I told you that one of the earliest Christian scholars to encounter Islam face-to-face—a Church Father, no less—did exactly that? Not as an insult, mind you, but as a theological classification. Not to incite war, but to explain what he saw before him.

This is not about politics or polemics. It’s about theology, history, and how ideas evolve. And my selfishness, really. I’d love to resolve the differences once and for all so I could travel with impunity!

Let’s take a deep dive.

Enter St. John of Damascus: Patron Saint of Pattern Recognition

Born in Damascus around 675 AD, St. John of Damascus lived under early Islamic rule and served in the court of the Caliph before retiring to a monastery near Jerusalem. He’s remembered for defending icons during the Iconoclastic Controversy, hymnography, and encyclopedic theological writings.

But a fascinating detail is tucked into his work “De Haeresibus” (On Heresies).

Islam, he writes, is the 100th heresy.

That’s right: in his list of heresies plaguing Christendom—from the Gnostics to the Arians to the Nestorians—John classifies Islam as a heresy derived from Christianity, not a new paganism.

His reasoning?

Islam accepts some Christian teachings—Jesus as a prophet, the Virgin Birth, and Mary’s holiness. But it denies key doctrines—the Trinity, the Incarnation, the Crucifixion, and the Resurrection.

From John’s perspective, this made Islam a partial Christian offshoot, much like Arianism or Monophysitism. Muhammad, he argued, cobbled together a new doctrine from Judaic monotheism and distorted Christian teachings, with a few Arabian folk traditions sprinkled in.

Belloc’s Bombshell: Islam as the Great Heresy

Fast-forward over a millennium. Enter Hilaire Belloc, the French-British Catholic historian, firebrand, and all-around contrarian. Writing in The Great Heresies (1938), Belloc doesn’t mince words:

Mohammedanism was, in the beginning, a heresy: that is, it was a perversion of the Christian religion. It began as a heresy, not as a new religion.

He goes further than John of Damascus. Belloc claims that Islam isn’t merely heretical—it’s the most dangerous heresy Christianity has ever faced. Why? Unlike other heresies, Islam created a civilization, wielded the sword, and commanded armies. It conquered Christian lands and ruled over Christian peoples. It wasn’t just theological—it was geopolitical.

Belloc admired Islam’s staying power. He warned that it could rise again—a prediction that now reads like prophecy in a post-9/11, post-ISIS, post-migrant-crisis world.

But was Belloc right?

Heresy: A Technical Definition

Let’s pause for a moment. What is a heresy, anyway?

According to the Catholic Church, heresy is “the obstinate post-baptismal denial of some truth which must be believed with divine and Catholic faith” (Canon Law, 751).

That definition is narrow. It presumes the person is baptized, was taught the faith, and then knowingly rejects part of it.

Islam doesn’t fit that mold easily. Muslims are not apostate Christians—they’re adherents of a separate religion, albeit one with striking theological overlaps with Christianity.

So while John and Belloc saw Islam as a heresy because it seemed to borrow and then twist Christian concepts, by today’s canonical definition, Islam isn’t a heresy.

It’s another religion.

The Vatican’s View: From Polemics to Parleys

Thanks to the recent conclave electing Pope Leo XIV, the Catholic Church has seen renewed interest. It’s also come a long way since the medieval polemics.

In 1965, during the Second Vatican Council, the Church issued Nostra Aetate, its declaration on non-Christian religions. It states:

The Church regards with esteem also the Muslims. They adore the one God… who has spoken to humanity; they endeavor to submit wholeheartedly to even His inscrutable decrees.

This wasn’t theological capitulation. It was a strategic shift—from confronting to engaging. The Church recognized the global reality: nearly two billion Muslims inhabit the planet. Many share moral values with Christians: family, modesty, fasting, almsgiving, and prayer.

The tone was now ecumenical, not adversarial.

But don’t mistake that for doctrinal agreement.

The Catechism of the Catholic Church still affirms that “the Church is necessary for salvation” (CCC 846), and that Jesus Christ is the only-begotten Son of God, not just a prophet.

The Church respects Islam. It doesn’t endorse its theology.

So… What Is Islam? A Heresy, a Rival, or a Sibling Faith?

Let’s review the three options:

1. A Heresy (St. John & Belloc’s View):

Islam emerged after Christianity and borrows heavily from Christian theology—Jesus, Mary, Scripture, eschatology—but distorts it.

Islam denies the Trinity and Incarnation, thus denying the heart of Christian faith. Therefore, it is a Christological heresy—like Arianism, but on a civilizational scale.

2. A Rival Religion (Modern Vatican View):

Islam is its own faith with its own prophet, scripture, laws, and theological framework. While it shares its origin with Christianity, Islam emerged as an independent religion. Although the Catholic Church acknowledges shared truths with Islam, it maintains clear theological distinctions.

3. A Sibling Faith (Interfaith Dialogue View):

Islam, Judaism, and Christianity are the three Abrahamic religions, highlighting their common ancestry and shared ethical principles.

This makes interfaith dialogue and peaceful coexistence easier to maintain. But it downplays significant and fundamental theological disagreements between them.

Why This Still Matters Today

This isn’t just academic banter or intellectual trivia. How the West views Islam has profound consequences—political, cultural, and even existential.

If Islam is a heresy, it can be debated, critiqued, and even evangelized with urgency.

If Islam is a separate but equal religion, it must be respected, but its truth claims must still be contested.

If Islam is merely a sibling faith with a few differences, Christianity’s unique claims lose gravity, and religious relativism wins the day.

The Church is trying to walk a tightrope. It wants to honor truth without inciting violence. It wants dialogue without dilution.

But clarity matters. If Christianity means anything, Jesus is more than a prophet, and the Trinity is not a negotiable mystery.

Islam says otherwise, which means the conversation isn’t over… by a long shot.

Wrap Up

The question of whether Islam is a heresy might sound medieval. That’s because it is—and thank God for that. The medievals believed words meant things. They drew lines. They defined terms. They didn’t fear the mob or the media.

We would do well to recover some of that courage.

An honest conversation, rather than the perpetual can-kicking, may solve something. It’s worth striving for, because despite what the legacy media says about this place, it’s worth getting to know better.