This Chapter introduces a new book-length study of the genesis and genius of the America’s constitutional commitment to granting religious freedom to all and religious establishments to none. The study analyzes the prescient movements for disestablishment in the colonies and the new states and the revolutionary new guarantees of religious freedom built into the First Amendment. It also shows how the ongoing battles over education, immigration, polygamy, funding, exemptions, and more have made the original and evolving understanding of the First Amendment disestablishment guarantee a source of perennial cultural and constitutional contest.