It's a Protestant thing. The meme basically goes all the way back to Martin Luther and the split from the Catholic church. Basically Protestants view Catholicism as a cult that idolizes and worships men and symbols, instead of worshiping god and the bible -- hence the meme that they're not "real" Christians.
It's pretty much Christianity's own version of the Shiite/Sunni split in Islam, and there have been plenty of bloody wars and massacres over it too. Fortunately they weren't sandniggers, so after a while they came to reason and made general peace over it so they're not currently blowing eachother up like the Muslims(except for some potato niggers up in Ireland anyways).