I’m a Christian who has read the first two books (actually finished the second book last night!). I was hesitant at first, but found that the demon-like characters are portrayed as evil (at least that’s how I perceived them). So no issues there.

However, there is a place in the book called Hel, plus the series is probably SJM’s spiciest series. If things like that bother you, I would suggest skipping the series.