Hmm, there are Christians who don't believe in Satan, though, and the show also implies that grace through faith is insufficient (otherwise the bishop would have been fine). Your assumption is based upon the idea that disbelief nets you a one-way ticket to Hell, which not everyone subscribes to, not even all Christians.
It's worth pointing out that the video games don't align with typical Christianity either; they have reincarnation in their plots and such. If the show decides to get into that, too, and if Mina is included and they make it clear that she is Lisa reincarnated (which, I mean... come on, that's what she is), then the subject of the afterlife becomes a bit murky.
Honestly, I don't think reincarnation is a huge stick in the mud for Christianity as a whole if you already acknowledge the existence of both miracles and the Resurrection of Christ.
While I personally disagree that Mina
must be a reincarnation of Lisa, there's nothing in the general Christian dogma that outright discounts it -- if one accepts that God is all knowing and all powerful, who are we to tell Him what He's allowed to do with his time, especially as it pertains to the redemption of the
ultimate lost and wayward soul?
I mean, I personally think that Mina could just as easily be someone Soma met, whether by chance, fate, or divine providence. And she happened to be the RIGHT someone. God may have ordained that the same process or force that caused Dracula's soul to recycle and be reborn as Soma Cruz do the same for Lisa. That being said: I feel that, since escaping the "cycle" of Dracula is Plot Element Number One of Soma's arc, having Lisa (a
major component of the cycle, if not
the most major) be tied up in that would actually sabotage the purpose of Soma's reincarnation. Letting go of the past is the biggest part of moving on, after all. This is why I think it serves the story arc a bit better to have Mina be the "someone new" that gets what's left of Dracula to move on from Lisa; now unchained from his past in every way, Soma is free to choose his path in a way Dracula wasn't able to.
Having Lisa involved in all of that just mucks up the storytelling, in my opinion. You CAN tell the same story with that, but I just think it gets needlessly complicated and a little less beautiful/meaningful.
But then again, I'm not about to tell the Big G how to do His business either.