Ah, but have you ever been in a post apocalyptic world, or been born into one?
we'd all like to think we'd stay high and mighty or sensible, but somehow I feel my knowledge of classical civilisation I possess in real life would be slightly non-exsistant If I lived in a world of death and barbarity.
to true, but if it was me personally as I am now that was plonked into a wasteland, religion would more than likely be one of the last things on my mind I might eventually be persuaded to follow it but I wouldn't go looking for enlightenment, however if I was raised in the wasteland it would all depend on how I was raised.
If I was raised by raiders I wouldn't have a care for religion, however if born and raised in Rivet City I would probably at the least be accepting of religion because I was brought up in an environment that practices it.
But as you say (in a way) we don't know for sure.
edit:
And the Master wasn't entirely insane in his actions, from the little I know about the original Fallouts. His idea had SOME reasoning/logic behind them, he just didn't know mutants are sterile and such. And was, to say the least, a bit extreme.
that's due to our hindsight, it doesn't look bad and in some ways could have worked (apart from being sterile and a few other things)
Spoiler although for the people who lived during that time they would have burned the children of the cathedral if they knew what the master was doing and that they were harbouring the master.