I guess it really doesn't make a huge difference, Lyn, who made him a bishop. Either the TC or a high level Holder probably has the ability to get him in trouble if he actually registers on their radar, so both could make his life difficult. It isn't a bad idea to look into the larger Holder society. I've been playing him as sort of distant from all of that, but it might give me another facet to work an angle from.
As for Steff, I'm also a fairly religious person, but I've got a really bad opinion of churches (that gets complicated). I'd say that if you want to play an Amarr, just jump in. For all the back and forth going on here, very few people have brought it up IC and the only one of those that was Amarrian was Slaver Filth (who would have questioned Constantin's background even if he was airtight, it's just the character). The Amarrians ICly tend to debate and wax lyrical on Scripture and religion fairly harmoniously, considering we don't really have much more than a handful of quotes from their massive volumes of Scripture. You don't need to read too much. Just about the Holders, Privy Council, Theology Council, Moral Reforms, and of course Amarr Empire. I went as in-depth as I could looking through EVElopedia, but if I'd known what to look for it would have taken me about a half hour to get all the necessary information.
Just try not to step on too many toes and remember that the Amarrian faith isn't an IRL faith per se. I think if you want to play Amarr, having a background in IRL faith will help you by giving you a lot of perspective on how religion really plays a role in your life. It will definitely help you play the character organically rather than as a caricature of how someone might think a religious person thinks.