To believe in the inevitability of anything is to have a doctrine of history. To deny that one has a doctrine of history and yet to hold to the inevitability of anything is to take back with one hand what you gave with the other.
Secularism, the idea that a civilization can function for an extended length of time without any express religious commitments, is certainly an idea. And while secularists do not claim dogmatic inevitability for their secular arrangement in any overt way, they certainly like to act like the whole thing is inevitable. This assumption comes out if anybody has the bad faith to deny the inevitability of secularism. Anybody who denies that inevitability is a galoot, a bumpkin, a cornpone, a believer in Jesus.
By “believer in Jesus,” I mean someone who believes in Jesus, as opposed to believing in a theological definition locked up in a seminary somewhere. Even a conservative seminary. Especially a conservative seminary.
But secularism is an idea, and ideas come and go. Bad ones usually go. So the question is this: is there any basis for believing that secularism is inevitable, without that belief arising from an antecedent commitment to secularism?
No, there isn’t, you say? I didn’t think so. This means that the proposal of a “mere Christendom” needs to be answered on the merits of the case, and not because it is a relic of a supposedly outdated past.