Eugene Volokh On Bryan Fischer's Erroneous Claim:

Here. EV makes an important point. I don't mean to keep beating the dead strawman. But Fischer makes one key error common among Christian America types: That the original Constitution protected "Christianity" and excluded other religions. The problem is, the Constitution doesn't say that; it says it protects "religion" not "Christianity." And there is evidence -- EV reproduces it -- that the Founders understood "religion" meant more than just "Christianity." Finally, the First Amendment uses the term "religion" in two clauses like a Siamese twin that shares one heart. It's logically impossible for the term "religion" to mean one thing for Free Exercise purposes and another for Establishment Clause purposes. The "thereof" in the FEC relates back to the term "religion" as used in the EC.