Americans United for Separation of Church and State | Americans United for Separation of Church and, Americans United for Separation of Church and State (Americans United or AU for short) is a 501(c)(3) nonprofit organization that advocates separation of church and state, a legal doctrine set forth in the Establishment Clause of the First Amendment to the United States Constitution, which says "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof.".