Because if you just take one, he'll drink all your beer.
WAA-TAA!
because they're filled with the Holy Spirit
A scarfish
A: A fish!
He buys two cases of beer instead of one.
The designated driver.
For me, it's gotta be Emo Phillips' Baptist joke. Although to tell it you've gotta get his execution down.
A Catholic will say hello when he sees you in the liquor store.