I think that a big part of the philosophy of the GOP base is an unconscious belief that, other than a handful of sinners, everyone is Just Like Them. You hear it over and over in the voter ID debates: "Well, everybody has a state issued ID!" (No, they don't.) Same idea in play here: "Well, everybody is a Christian!"
Republican Horrified to Discover that Christianity is Not the Only Religion (Jezebel)
It's an honest mistake, assuming that the Constitution only protects your own personal megachurch faith. But one Louisiana Republican is learning the hard way that religious school vouchers can be used to fund education at all sorts of religious schools, even Muslim ones. And while she's totally in favor of taxpayer money being used to pay for kids to go to Christian schools, she's willing to put a stop to the entire program if Muslim schools are going to be involved.
Valarie Hodges admitted that when she supported Governor Bobby Jindal's school voucher program, she only did so because she assumed the religious school vouchers could only be used for Christian schools. Religious freedom means that everyone's free to follow Valarie Hodges' religion! She explains,
"I actually support funding for teaching the fundamentals of America's Founding Fathers' religion, which is Christianity, in public schools or private schools. I liked the idea of giving parents the option of sending their children to a public school or a Christian school."