Justin Welby
Justin Welby … 'Public school evangelicals can sometimes be human if they are grand enough. In practice, this means Etonians.' Photograph: Owen Humphreys/PA

People have been treating Justin Welby's Etonian background as a drawback for his role as archbishop of Canterbury. How can a child of privilege understand the poor and miserable? Actually, public schools are astonishingly good at producing miserable children, even if they can't make them poor. One reason why there are so many fervent atheists in the English upper middle classes is that they were confined at school with fervent Christians. Public school Christianity is a religion for the people who do well in high school – those whom Americans would call jocks. They play rugger, they sing hymns, they're chosen as prefects.

Perhaps because I went to a second-rate public school myself for a while I have always detested public school Christianity. Lindsay Anderson's film If, set in a school like this, contains a climactic scene where the heroes machine-gun parents' day from the roof of the chapel. When I first saw this, as a young man, I was delighted. When I remember it, I see the chapel and surroundings of my own old school.

Marlborough, Repton (where the former archbishop Geoffrey Fisher was a headmaster), Rugby (home of the "Christian gentleman" and Harry Flashman) – all these ghastly Victorian foundations produce strapping men who'd make first-rate leaders of cannon fodder in the trenches, but in later life they treat their own feelings with brutal disdain, much as they'd handle servants demanding a wage rise.

There is only one exception to this rule. Public school evangelicals can sometimes be human if they are grand enough. In practice, this means Etonians. I don't think I have ever met a Wykehamist Christian – presumably they think themselves too clever, though prepared to tutor God privately if they needed the money – but Eton is such a hothouse that no one comes out of it without a sneaking suspicion that he may not quite be the crown of creation. In some of them this appears as a restless and devouring ambition. In others, this insecurity translates into a genuine humility rather than the modesty that all Etonians affect so well.

John Habgood, the last Etonian archbishop, is a case in point. Perhaps this is because he was at heart a nerd, a scientist with a Cambridge PhD, one of the bulliable classes whose life would have been hell in heartier schools had he not been so tall. This gave him a rather cold mien but that was scientific rather than Etonian. He always seemed to me shy, serious and oddly vulnerable.

There is one other possible advantage to a Christian in attending Eton. It really would be impossible to graduate from there with a starry-eyed view of human nature. Nothing much shocks an Etonian because they have seen it all before the sixth form – they expect treachery and ambition to be the human norm (si monumentum requiris, Boris Johnson). And if God can redeem an Etonian, he can redeem anyone.