Normally, I like mysteries connected to religion. From Chesterton's Father Brown to the wonderful works of Father Andrew Greeley or the adventures of Rabbi Small, there's just something in the mixture that usually works for me.
Unfortunately, that was not the case with Saints Preserve Us. First of all, the mystery itself was not particularly well done, with a solution far too obvious for experienced readers. And second, the reason why so many religion-connected mysteries work for me is that the solvers use their knowledge of religion and experience of human nature to come to the bottom of the story. In this case, the main character showed neither, and the religion bits themselves felt preachy and silly.
I must confess I stopped reading at around 100 page mark, and only leafed through to the end to check my solution. I had guessed right, but that was hardly surprising. All in all, a disapppointing book.