Origins

It's foundational, but the phrase "separation of church and state" never actually appears in the U.S. Constitution. It was written by Thomas Jefferson in a letter to the Danbury Baptist Association. Key quote:

...that their legislature should make no law respecting an establishment of religion, or prohibiting the free exercise thereof, thus building a wall of separation between church and state.

The author's claim that Jefferson was a man of deep religious conviction is still in dispute. But you can read the letter here.