I begin every novel with the vow that I will not write about technology, Catholicism, or hell. As you know, I end up writing about all three. They just happen to be personal obsessions of mine. However, the next novel will not deal with any of them.
The most interesting character to me is someone who is stuck in the no man's land between Belief and Unbelief, Faith and Faithlessness. I'm capitalizing like a German, but it doesn't matter whether it's faith in a person or in God, or belief in science or whatever, it's the desperate in-between state that makes for interesting dramatic tension.
As for most Catholic novel to date? Mmm. Chapter 12 in White Man's Grave, where one of the protagonists goes to morning mass and confession for the first time in 40 years, is definitely my most Catholic chapter, but perhaps I might agree that in terms of direct references to Catholicism (as opposed to belief in the soul or Christianity), Bet Your Life has the most, simply because the main characters are all Catholics or fallen-away types.