By RAY WADDLE New York Times News Service Garrison Keillor has a pretty good thing going on "A Prairie Home Companion." All he has to do is say "Lutheran," and he gets a laugh. He says "Unitarian" and gets a bigger laugh. These words are not in themselves laughable. But over the national airwaves, it's different. Keillor knows that in the congregational setting of his secular radio show, the lingo of church life has a strange charm. Comic memories accrue and detonate with each new reference to "Corinthians" or "Episcopalians" in the tales of the show's stoic, church-going Minnesota characters....