From a recent Pew Survey on U.S. Religious Knowledge, atheists and agnostics know more about religion than most religionists. Atheists and agnostics score particularly well on knowing something about world religions but also do better than most on Christianity. The effect remains even after controlling for education.
Addendum: The Atlantic has a good run down on commentary. I was especially amused by religion scholar Stephen Prothero's line "those who think religion is a con know more about it than those who think it is God's gift to humanity."