The separation of church and state is established by Article VI and the First Amendment of the United States Constitution and codified into legal precedent in the 1878 case of Reynolds v. United States. The First Amendment states that "Congress shall make no law respecting an establishment of religion or prohibiting the free exercise thereof" and, in the case of Reynolds v. United States, the Supreme Court ruled that this amendment granted that there should be a separation between church and state.
The separation between church and state is also established in the 14th amendment. This amendment was ratified after the Civil War to help issues that former slaves were having. This amendment states that all citizens will be treated equally under the law and has since been ruled to apply to people regardless of religion.