I wouldn’t care if a player wanted to pretend his PC was a ‘Christian’ or not. Back in the day one of the guys I played with had a cleric he named, “Father Francis the Franciscan.” An NPC cleric in my first game was a cleric at the Church of Saint Alphonzo (named after the church in a Frank Zappa Song). Another player wrote “Yahweh” at the top of his character sheet and when I asked him why, he said because that was the god his character worshipped. Half of our cleric minis had little crosses in their hands or hanging around their little lead necks (see image of the ‘Dungeon Dwellers” clerics at above right; I think three out of four are carrying or wearing crosses and the other one has an ankh — I still have most of those little guys. They also made an “evil cleric” mini and you could tell he was evil because he had a grimace on his face and was holding his cross upside down! Just like a satanist from a horror movie!). We didn’t delve too deeply into what form their prayers or observances took (no, we did not try to really cast spells beyond saying, “My character will cast Magic Missile at the troll!” and the like).
In the first version of the rules I owned, the price list included things like “wooden cross, silver cross,” etc., until it was later replaced in the newer editions by the more P.C. term, “holy symbol.” There was holy water in the rules and a reference to killing a vampire by filling it’s mouth with holy wafers and the pictures of clerics in the books sometimes looked like Friar Tuck or was wearing a cassock, surplice or mitre or swinging incense censers or holding chalices (admittedly, I now know the mitre was also worn by Babylonian priests)… so, to this former altar boy, much of the trappings of ‘make believe’ D&D religion came from the real world Christianity and it didn’t bother me (and I considered myself an observant Catholic at that time). I just didn’t see the harm in having the references to real world religions in fantasy, and, honestly, I still don’t — I think most of that stuff was excised to be more P.C. in the wake of “D&D is devil worship!” scandals and Geraldo Rivera style “journalism.”