The historical document that formalized the separation of church and state in the U.S. is the First Amendment to the Constitution, ratified in 1791. This amendment prohibits Congress from establishing an official religion or impeding the free exercise of religion, enshrining the principle of religious freedom in American law. The separation of church and state was a significant departure from many European nations, where church and government were often intertwined. This principle is fundamental to American democracy, fostering a diverse, pluralistic society where individuals can practice their beliefs freely without state interference, thereby ensuring a framework for religious liberty that has influenced other democratic countries.