Actually the Klan
required that any incoming member be a Christian, and specifically a Protestant one.
The big Klan, the one we have all the pictures of, the one that spread nationwide, was founded by an ex-Methodist minister, using a bible, an unsheathed sword and an American flag. Stone Mountain Georgia, Thanksgiving 1915.
It's right here on the application forms.
"Christian terrorism" if you like. Their targets included Jews, Catholics, immigrants in general, labor unions, blacks, drunks (Klan were strongly pro-Prohibition) and adulterers, philanderers and "loose women". I call 'em a Christian Taliban.
Klanners would often walk into church services, in full regalia, and make donations.
There was at least one occasion they pulled a (white) woman out of her house and whipped her for the 'crime' of "not going to church". When her fifteen year old son came out to defend her, they whipped him too. They were heavy into the flagellation thing.