You know, the one that reads "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof"?
Because Christianity deems a great many things wrong that are accepted as commonplace behaviors in the modern world. Things like coveting anything that is thy neighbor's to the point that, if you can't have it (or something like it) you either get the government to steal his wealth to redistribute it to those who didn't earn it, or destroy it. Or sleeping around outside of marriage. Or blaspheming (which I define as doing evil in the name of your so-called "good" deity).
Or acting on homosexual urges.
Over in Britain, a Baptist preacher was arrested for discussing what the bible defines as sinful. They can't do that over here, yet, and it's thanks to that first amendment. The Brits have nothing like that protecting their God-given rights.
1 hour ago