Freedom of religion in the United States doesn’t mean you may believe in anything or nothing. It only means that you’re free to choose between Christian denominations.
[T]he purpose of the First Amendment, as Justice Joseph Story declares in his monumental history of the Constitution, was only to protect the free exercise of the Christian faith and to prevent the selection and designation of one Christian denomination as the official church of the United States.
In other words, you can be any religion you want, as long as it’s Christianity. More importantly, the government may promote one faith over all others (three guesses which one they supposedly picked).