VT Hawkeye makes a good point when he says that freedom of religion is not the same thing as freedom from religion. However, his underlying assumption is dead wrong. There is no Constitutional guarantee of freedom of religion.
To understand why this is, consider the worst-case scenario. If I had a constitutional guarantee of freedom of religion, I could start my own Cult of Mr. T, in which the First Commandment would be "Thou Shalt Kill." The Supreme Court, guided by the Consitution, would be compelled to rule in my cult's favor and protect our right to kill whomever we pleased. This would not be a good thing.
In real life, of course, this never happens. Rastafarians, for example have their freedom of religion trampled upon (and unfairly, in my opinion) because their religious practices violate our laws. The reason this can happen is because of the actual wording of the part of the First Amendment that deals with religion. "Congress shall make no law regarding the establishment of religion." In other words, it is imperative upon our government to strip all religious overtones and influences from its actions. In the United States of America, as far as the government is concerned, we all have the inviolable right to freedom from religion. Period.