Actually, what the First Amendment says about religion is:

Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof.

How such a seemingly clear and simple sentence can be the source of so much controversy is, in part, bound up the the ambiguity of the words "respecting" and "establishment". "Respecting" can mean "holding in high regard" or "concerning". "Establishment" can mean the act of establishing, or it can mean a physical building (a church is an "establishment of religion"), or an organization such as a religious denomination (the Episcopal Church is an "establishment of religion").

So, does "Congress shall make no law respecting an establishment of religion" mean that there will be no establishment of an official national religion? Or does it mean that no particular religion will be held in higher regard than any other? Or does it mean that Congress shall keep its hands off of religion entirely? Either of those interpretations is valid, depending upon what exact meanings of "respecting" and "establishment" one adopts.