Two rooms, in two different cities, but pretty much the same scene: one man stands before a few dozen supporters, many of them middle-aged white males, plus a smaller, precocious cohort in early adulthood. As the man speaks, they interrupt him with good, earnest, detailed questions, which he ably answers more or less to their satisfaction. These crowds crave the intricacies of arguments and the upshots of science. The only thing that seems beyond their ken is how their counterparts in the other room could be convinced of something so wrong.

One of those rooms was in New York City, high in an office building overlooking the ruins that then still remained of the World Trade Center; the man was Richard Dawkins, the Oxford zoologist and ‘New Atheist’ polemicist. The man in the other room was his arch-rival, the evangelical Christian philosopher and debater William Lane Craig, speaking in a classroom on the sprawling campus of his megachurch in Marietta, Georgia. If one were to attend both events without understanding English, it would be hard to know the difference.

Whether such a thing as God exists is one of those questions that we use to mark our identities, choose our friends, and divide our families. But there are also moments when the question starts to seem suspect, or only partly useful. Once, backstage before a sold-out debate at the University of Notre Dame between Craig and Sam Harris, Dawkins’s fellow New Atheist, I heard an elderly Catholic theologian approach Harris and spit out: ‘I agree with you more than I do with that guy!’

During the heyday of the New Atheist movement, a few years after the terrorist attacks of 11 September 2001, I was in the wake of a teenage conversion to Catholicism. One might think that my converts’ zeal would pit me squarely against the New Atheist camp. But it didn’t. Really, neither side of the does-God-exist debates seemed to represent me, and the arguments in question had little to do with my embrace of my new-found faith. I had been drawn by the loosey-goosey proposition that love can conquer hate and death, expressed concretely in the lives of monks I had briefly lived among and members of the Catholic Worker Movement who shared their homes with the homeless and abandoned. I actually agreed with most of what the New Atheists wrote about science and free enquiry; what I disagreed most sorely with them about was their hawkish support for military invasions in Muslim-majority countries.

Still, I became fascinated with the question of God as I tried to wrap my head around it for myself. I travelled around the world to meet God debaters, and studied the historical thinkers from whom their arguments derive. I found that I wasn’t alone in doubting the pertinence of the question.

The thinkers who crafted the classic proofs for the existence of God – from Aristotle to Thomas Aquinas, for instance – were writing to audiences for whom the existence of divine beings was uncontroversial. The purposes of these proofs had more to do with contentions about what we mean by God, and how far into such matters human reason can really take us.

Consider, for instance, Anselm of Canterbury, an 11th-century monk who devised his proof in a fit of early morning ecstasy. His claim, which has been debated strenuously from its first publication until now, was that the very concept of God contained in it the proof of God’s existence – which, to Anselm, was a testament to God’s omnipresence and love. For centuries, his fiercest critics objected not to Anselm’s God, but to his reasoning. Centuries later, the Jewish apostate Baruch Spinoza employed a very similar argument in 17th-century Holland: he took the reasoning but mostly put aside the God.

Today, Spinoza stands as a progenitor of the modern, scientific worldview. The atheist philosopher and novelist Rebecca Newberger Goldstein considers him ‘the renegade Jew who gave us modernity’. Yet at the centre of his system is a proof for God, one very much akin to that of the Christian monk Anselm. Where Anselm saw the Christian God, Spinoza saw the totality of the universe. He insisted that this was indeed God, that he was not an atheist. In his devotion to reason, Spinoza became famous for his piety; the German Romantic poet Novalis would later call him the ‘God-intoxicated man’.

Spinoza and Anselm both passionately believed in God, and adopted a similar way of thinking; the difference was in the kind of God they had in mind.

In the 20th century, the novelist and philosopher Iris Murdoch would take up their basic argument again. She saw in it neither Anselm’s God the Father nor Spinoza’s God of Nature, but the Good – the underpinning of morality and beauty in a post-religious world. When we compare her to Anselm and Spinoza, the question of God-or-no-God seems far less interesting than the argument they shared and the ways in which they tweaked its meaning. I wonder what Anselm and Murdoch would say to each other if they were to somehow meet.

What are we really talking about when we debate the existence of God? I think it can become a shortcut, a way of side-stepping more necessary and more difficult questions. Denouncing others as atheists, or as believers in a false God, can become an excuse to treat them as less than human, as undeserving of real consideration. When terrorists attack in the name of a certain God, it can seem easier to blame their religion than to consider their stated grievances about foreign military bases in their countries and foreigners backing their corrupt leaders. When religious communities reject scientific theories for bad reasons, it can seem easier to blame the fact that they believe in God, rather than to notice that other believers might accept the same theories for good reasons. Good ideas and bad ideas, good actions and bad actions – they’re all on either side of the God divide.

Pope Francis’s provocations in recent years have been palpable reminders of this. When Francis released his recent encyclical on ecology, many non-religious environmentalists received it more warmly than some of my fellow Catholics. Francis himself addressed the document not merely to Catholics, but to ‘all people’, and he has welcomed secular activists to the Vatican to discuss it. (The journalist Naomi Klein was so enthusiastic upon returning, she told me, that she had to remind herself ‘not to drink too much Kool-Aid’.) Meanwhile, the conservative Catholic blogger Maureen Mullarkey dismissed it as an ‘extravagant rant’. Catholic friends of mine found it depressing, while I read it by a lake with tears of joy. The fact that we share a belief in the God that Francis calls upon was, for better or worse, beside the point.

I believe in God, but I often find more common cause with those who say they don’t than those who say they do. I’ve come to care less whether anyone says they believe in God or not, and to care more about what they mean by that, and what they do about it.