Lyric discussion by zhe-neech 

Cover art for Modern Love lyrics by David Bowie

This song has nothing to do with religion. It's about marriage, and love. He contradicts himself on purpose, in order to demonstrate what "modern love" is, which is a changing thing. He believes in it, and he doesn't. He believes he'll get married whilst thinking that it's impossible because of modern love.

@zhe-neech your interpretation is the closest to what I feel it may be.