In other words, you mean a book with characters who are Christian but whose stories take place in a world where God isn’t real in any literal way?

Assuming this is what you mean, then no, there isn’t really an audience looking for this type of story.

In fact, many people in the secular world will actively avoid it. These reluctant readers would likely require some convincing via accolades / awards.

A story like this will succeed or fail entirely on other specific genre elements.

Is it scary enough (horror/thriller), wise enough (literary), or clever enough (mystery)?

Basically, the religious aspect of such a book would be a caveat if it succeeds, not the cause.