Do you think a novel about a Christian-LaVeyan Satanist (atheist, not actual Devil-worshiper) romance might interest people, or would it more likely seem gimmick> How could I make it not seem gimmicky?
Truthfully, my eyes automatically did an upward roll upon reading that combination. Unless you're wanting to get into the whole Anton LeVay shock and awe school of misleading titles, why not just go with a Christian and a plain old atheist?
Don't make it the main point of the story. Have them be complex people, with more going on in their lives than just what club they belong to or what belief system they have. Obviously they must be able to see past those things in each other or they wouldn't have a relationship (unless one was ready to dump their own belief system).
Everyone loved these two together in Stranger Things (Spoiler Alert: they were NOT a couple but every one shipped them together after 1 single scene of him making her laugh) But everyone like the possibility of a relationship between these two: Chrissy: good Christian girl dating a good Christian guy questions/really down on herself trying to be perfect cheerleader repressed angst Eddy: D&D dork "Satanist" IDGAF attitude... ...yet caring to a fault goofy emotional They were opposites. She, like everyone else, didnt like Eddy/was afraid of him because they thought he was a devil worshiper, but they got to talking and formed somewhat of a friendship (mutual acceptance?) So i would definitely say people would ride that wave if you wrote it... an if you wrote it well. Like @Xoic said, give them their own stories apart from each other and then have their lives intersect somehow. A mutual connection/bond. It could even be the "big bad scary atheist made cheered me up when i was feeling low." Will their beliefs play a very significant role in the novel? Or is it just background to cause a bit of conflict/tension
Aside from what I said earlier (one is ready to give up their belief system) it could be that they're just strongly attracted to each other, so much that their conflicting beliefs don't matter. It causes both of them intense anguish—"How could I feel this way about somebody who believes the opposite of what I do?" But still, if you're a Christian and becoming romantically attached to a Satanist, or vice verse, it calls into question whether you really believe in what you claim to believe, or have any real commitment to that belief. Is there any more depth to this than just 'Christian and Satanist get involved in a romance'? If that's the whole thing, then it's a gimmick, and I don't think you can make it not feel like one. To not be a gimmick it would start with some more depth to it—they'd have more dimension to them than just their religious beliefs. You haven't said anything about them as people, about their personalities or inner character, their other beliefs, etc. Or it could be a wild comedy.
As a Christian, I find the idea so ridiculous that I thought you were writing the idea to make fun of Romeo-and-Juliet-type romances. I actually laughed out loud when I saw the title. That’s not meant as an insult; you could write it as humor. But seriously, I just can’t see how it would work out. I can tell you this from experience: people think we Christians don’t like unbelievers, but the reality is that they’re the ones who dump me before I even have a chance to decide whether I like them. If you’re going to write this, I suggest talking to a real live believer in each of the two religions.