[skissane]

I doubt non-religious people are ever going to die out completely.

But, it is plausible they are going to transition from being the population majority (in most rich countries) to being a minority. That is going to challenge popular narratives of secularism's "historical victory"

What if the eventual triumph of pro-fecundity religious conservatism results in runaway population growth? You can't fit 1 trillion humans on this planet, and it would be a living hell even if you could–but space colonisation might turn out to be a massive escape valve. In such a scenario, it is possible we could end up with secular people being a small minority of the total human population, yet simultaneously many times more of them (in absolute numbers) than currently exist.