'"legislature" should "make no law respecting an establishment of religion, or prohibiting the free exercise thereof," thus building a wall of separation between church and State.' Thomas Jefferson
Jefferson had to deal with the settlers of the America being of a mix of religions, Catholic, Puritan, Quaker and others. And also he was aware that declaring one faith official and persecuting the others had caused problems in Europe. This had a lot to do with bringing in the separation of church and state. And with preventing the government prohibiting freedom of religion.