"Lost Christianity" by Jacob Needleman
Some of the passages in this book are over my head - meaning long, involved sentences of philosophy too deep for me to fathom.
But there are some interesting ideas put forth about what is lacking in modern Christianity that drives its followers to look for "whatever it is" elsewhere (such as paganism, Buddhism, etc.) I'm going to have to read it again in order to absorb some of it, but I have passages highlighted all over the place that resonate with me.
He has some fascinating ideas about how Christian ritual has become a meaningless exercise that is more about the ego, when in reality ritual was meant as a way to release the ego and listen for that "still small voice". He also talks about the concept of "spiritual psychology" which explores what it is about religion/ritual that feeds something that is hard-wired for us as humans to need. (Actually Needleman is relating ideas and writings of four men he interviewed over several years' time.)
Anyway, like I said, I need to read it again. There's a lot in there to absorb.