The Bloudy Tenent of Persecution, for Cause of Conscience, Discussed in a Conference between Truth and Peace was written by Puritan leader Roger Williams in 1644. The Tenent is one of the earliest colonial writings advocating for a "Wall of Separation" between church and state, and for deep toleration by government of Christian beliefs.
Early advocates of the Separation of church and state in what believed what could better be described as a separation between denomination and state.