Indeed, it’s completely obvious that absent the Christian faith, there would be no liberalism at all. No ideal of universal human rights without Jesus’ radical upending of social hierarchies (including his death alongside common criminals on the cross). No separation of church and state without the gospels’ “render unto Caesar” and St. Augustine’s two cities. No liberal confidence about the march of historical progress without the Judeo-Christian interpretation of history as an unfolding story rather than an endlessly repeating wheel.Larison notes that the progress of liberalism has indeed gone hand in hand with the development of our civilisation, which until recently has been explicitly Christian. It would be pretty hard, I think, to argue liberalism in the exact form we currently recognise it (to the extent that such nebulous concepts can be described as "exactly" anything) would have evolved without Christian influence.
It's much harder, to put it mildly, to believe that absent Christianity, liberalism in some form wouldn't exist. Indeed, Douthat's argument isn't "completely obvious", it's somewhere between a completely unproveable counter-factual and an assertion which is absurd on its face.
Whenever an atheist argues that without Christianity there'd have been no Crusades, no Inquisition, and no Nazi Party, it pisses me off. The human desire for power, wealth, and the subjugation and hence neutralisation of the "other" is sufficiently ingrained in our lizard brains for it to be easily arguable that Christianity has provided an excuse for atrocity, not been the cause. At least some of the Crusades were undertaken for no better reason than the Holy Church wanted more power, and whilst the glib (and common) response to that is to point out that, yes, the church is explicity Christian, the concentration of so much power in the hands of so few on the grounds that God wants it that way is just further evidence that religion can be applied as a tool by some very, very bad people.
Douthat's claim seems to be the mirror image of that approach. Those who forged the philosophy of liberalism did so through reference to the Bible, therefore the Bible deserves the credit.
Consider what would have happened were Christianity removed from world history. Would the West be atheist from coast to coast? This seems vanishingly unlikely, given the way religions spread. I'd assume we'd be Muslims, or possibly Hindu. Does Douthat really want to argue that universal human rights aren't something any other religion could conceive of? Does he really want to tell the descendents of Gandhi that he owes his view of the universal dignity of man to the religion of his colonial oppressors? Really? Even the famously peaceful Gandhi would have wanted to tell him to fuck off for that one, I'd have thought. Ditto the tens of thousands of human rights activists in jail across the world right now, an awful lot of them who aren't Christian, and would be fairly outraged to learn they owe their deep convictions to Jesus.
I realise that Douthat is making these comments in the middle of a conversation about the American approach to liberalism, but that's precisely why his sweeping generalisations are so problematic; he's writing off the entirety of non-Western culture as being philosophically incapable of even conceiving of human rights or the separation of church and state. It's that latter point, by the way, that confirms he's insisting these ideas are generated by Christianity specifically, and not religion in general, since without religion of any kind there indeed wouldn't be a concept of separation of church and state, for the same reason there'd be no concept of anti-aircraft guns without anyone ever having built a flying machine.
I'd actually really like to see a consideration of how a society without any kind of religion could generate what for shorthand I'll call humanist principles. That isn't what Douthat is doing, though. He's claiming Western civilisation has a copyright on a decidedly global concern, and in the process arguing that those who for so long were oppressed by Christians could only conceive of their right to be free because of the religion their oppressors brought with them. Nice.