The early American advocate for the separation of church and state was Thomas Jefferson. He articulated this principle in his letter to the Danbury Baptists in 1802, emphasizing the need for a "wall of separation" between religion and government. Jefferson believed that this separation was essential for ensuring religious freedom and preventing the government from favoring one religion over another. His ideas played a foundational role in the development of the First Amendment of the United States Constitution, which guarantees freedom of religion and prohibits the government from establishing a religion.