When I was in college, I belonged to the campus role-playing group
(the Miskatonic Student Union). Of course, we had frequent run-ins with the
Campus Crusade for Christ, who thought we were devil worshippers. So we had
t-shirts made up:
On the front:
"Campus Crusade for Cthulhu" (w/ picture of Cthulhu)
On the back:
"Our god can eat your god for breakfast!"