Actually he is a baptized Catholic, but chose to join the Mormon Church to please his wife.
Poooooorrrrrr Glenn.
That whole thing always bugged me. By all accounts, his in laws are Italian to the core i.e.; Catholics. What goes unnoticed is the fact that Pat Grey is a funny underwear wearer from way back and Glens best friend. I think that when Glenn was drying out, this dude told him that Mormonism was where it was at and he bought into it. Glen is pretty slow when it comes to push and shove.
"I won't marry you unless we have a religion we share." (timeline 0:58)
http://www.youtube.com/watch?v=USpeolBTKIo
(Evidently SOMETHING about Catholicism didn't appeal to her.)