Books with religious undertones

I'm reading The Plague right now and there was this small part that intrigued me

"Many centuries ago the Christians of Abyssinia saw in the plague a sure and God-sent means of winning eternal life. Those who were not yet stricken wrapped round them sheets in which men had died of plague, so as to make sure of their death."

I'm not religious at all as a person but I find fascinating what humans can do because of their faith. I also love themes about their internal struggle while they doubt if God actually exists. I think these types of characters can be very one-dimensional and very very complex at the same time.

I read Siddhartha in the past and although I found it alright, it's not exactly what I'm looking for. I also definitely don't want something preachy either.

Two books I think I'd like are Silence by Shūsaku Endō and The Leftovers by Tom Perrotta. I haven't read either but I've seen the film and the tv show so I'm judging based on that.