"Oh, you have the revised Constitution that has that infamous "separation of church and state" thing."
The phrase "separation of church and state" is derived from a letter written by Thomas Jefferson to a group identifying themselves as the Danbury Baptists. In that letter, quoting the First Amendment of the United States Constitution, he writes: "I contemplate with sovereign reverence that act of the whole American people which declared 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."
Another early user of the term was James Madison, the principal drafter of the United States Bill of Rights, who often wrote of "total separation of the church from the state" (1819 letter to Robert Walsh). "Strongly guarded . . . is the separation between religion and government in the Constitution of the United States," Madison wrote, and he declared, "practical distinction between Religion and Civil Government is essential to the purity of both, and as guaranteed by the Constitution of the United States" (1811 letter to Baptist Churches). This attitude is further reflected in the Virginia Statute for Religious Freedom, which was originally authored by Thomas Jefferson, but championed by Madison. The Declaration guarantees that no one may be compelled to finance any religion or denomination.
It is not in the Constitution. States such as Massachusetts and Maryland both had religious -- religious sect, even -- natures to their laws and charters at the time of Constitutional ratification.