“No, the God of the Qur’an is Not the God of the Bible,” proclaims the headline of a recent essay at First Things by Gerald R. McDermott, a professor of religion at Lutheran-affiliated Roanoke College.

Staging the question against the backdrop of the 9/11 attacks, McDermott takes issue with the “nuanced but insistent Yes” of a book called Allah: A Christian Response by Yale theologian Miroslav Volf. Volf’s “argument for the identity of the Muslim and Christian Gods collapses under its own weight,” McDermott proclaims.

McDermott’s approach seems problematic to me from a number of perspectives. The very fact that he speaks of “the Muslim and Christian Gods” should be a red flag to any monotheist.

“God has no plural,” C. S. Lewis usefully wrote. It makes sense to speak of “the Muslim and Christian conceptions of God,” or perhaps even “the Muslim God and the Christian God”; but as soon as we start to speak of “Gods,” it’s a sign that our thinking may have gone awry.

False beliefs about God are one thing. Belief in a false god is something else. In that regard, it’s worth noting that while McDermott equivocates at first, speaking of the question of the “identity of the God or gods of these two religions,” he consistently speaks of of “the God of the Quran,” not “the god of the Quran” (all emphasis added).

Unless we wish to argue that the divinity of the Quran is a false god, a god who is no god at all, then it seems to me that we must acknowledge at least some sort of identity between “the God of the Quran” and what we should call “the true God,” or, speaking as Christians, “the God of the Bible.”

In that case, as Christians we ought to say that while we believe Quran includes both true and false ideas about God, they are ideas about God, not ideas about someone or something else, a false god. To that extent, the God of the Quran is the true God.

Why can’t we call the God of the Quran a false god? How can we tell when someone is talking about the true God, rather than some other god?

The question is obviously more than the mere English convention of capitalization; someone could choose to write about “the Gods of Egypt,” and that wouldn’t change the fact that they were writing about false gods, not the true God. What is the essential difference?

Consider what St. Paul writes in Romans 1:19-21 concerning pagans:

For what can be known about God is plain to them, because God has shown it to them. Ever since the creation of the world his invisible nature, namely, his eternal power and deity, has been clearly perceived in the things that have been made. So they are without excuse; for although they knew God they did not honor him as God or give thanks to him, but they became futile in their thinking and their senseless minds were darkened. Claiming to be wise, they became fools, and exchanged the glory of the immortal God for images resembling mortal man or birds or animals or reptiles … they exchanged the truth about God for a lie and worshiped and served the creature rather than the Creator, who is blessed for ever! Amen.

In this passage St. Paul tells us that human beings can to an extent perceive “the truth about God” from creation itself. In particular, God’s “eternal power and deity” are evident from the world. The cosmos doesn’t explain itself; it calls for a larger explanation. Continuing this line of thought, St. Thomas Aquinas argues in his “five ways” of proving God’s existence from various aspects of creation to a prime mover, a first cause, a source of perfection, etc., in each case concluding, “this we call God.”

This is perhaps the most basic formulation of the idea of God, as opposed to gods: the ultimate cause of everything else that exists. God is the ultimate explanation for why the cosmos exists, the transcendent answer to the question “Why is there anything rather than nothing?” The vast scope of the cosmos attests his great power; its intricate order and mysterious beauty attest his great wisdom and knowledge. Since he is the ultimate explanation of everything else that exists, we can also understand that, being uncaused himself, he is eternal.

This differentiates the idea of God in principle from the idea of other putative gods or divine beings, who also have foundational explanatory roles in the thought of their worshipers, but partial and relative ones, with various areas of competence or sway. Poseidon governed the sea and earthquakes; the harvest grew thanks to Demeter; and Zeus was responsible for weather, law and fate itself. But even Zeus wasn’t the ultimate cause of the world, and he had an origin of his own, being only the youngest son of Cronus and Rhea. Cronus and Rhea, in turn, were descendants of Gaia and Uranus.

Ultimately, in Greek mythology, all arose from chaos. But some pagan mythologies, pursued back far enough, acknowledge some divine being in some way above or prior to all the others, who is in some way the ultimate cause of all the others, and has no known cause himself.

Pagan religion might not tell us much about this high god; he may be remote and unknowable, far from human affairs. Still, this attempt to offer an ultimate divine explanation for all of reality at least approaches what we mean by God, though corrupted by the addition of other gods believed to be closer to men and more relevant to their day-to-day lives.

Read more: The Quran, the Bible and the Flying Spaghetti Monster