Does anyone else round here feel that their understanding of someone's personality is necessarily (or even usually) incomplete without some knowledge of their attitude to religion?

ObPedant: Yes. In that my knowledge of anyone's personality, even my own, is invariably incomplete, inconsistent and full of mistakes. Therefore it is trivially true that if I don't know someone's religion, I don't fully understand them.

That said, knowing someone's religion adds surprisingly little to my understanding of them. The way they approach their religion, maybe, but the datum that goes on the census form is surprisingly uninformative.