Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.So, as we see, O'Donnell was quite right. The phrase "separation of church and state" is nowhere to be found in the First Amendment. What is found in the text is a strong federalist approach to the question of religious establishment -- namely that Congress should not intervene to either set up a national religious establishment or tear down then-existing state establishments of religion. That congressional prohibition was later extended to the states via the 14th Amendment incorporation doctrine. But the actual texts of both the First or 14th Amendments do not employ the phrase "separation of church and state."
So, where does the phrase come from? As readers of this blog know, the phrase is not found in the Constitution or in the writing of any of the Founders at the time of the enactment or ratification of the Constitution or the Bill of Rights. The phrase comes from a letter written in 1802 by then-President Thomas Jefferson to the Danbury Baptist Association. The full text of the letter is available at the Library of Congress website, but here is the relevant language:
Believing with you that religion is a matter which lies solely between Man & his God, that he owes account to none other for his faith or his worship, that the legitimate powers of government reach actions only, & not opinions, 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 & State. Adhering to this expression of the supreme will of the nation in behalf of the rights of conscience, I shall see with sincere satisfaction the progress of those sentiments which tend to restore to man all his natural rights, convinced he has no natural right in opposition to his social duties.Jefferson's use of the phrase "wall of separation between Church & State," written well after the adoption of the Constitution and the Bill of Rights, is a key moment in the development of the idea that the First Amendment's guarantees of religious liberty and non-establishment entail the notion of separation between religion and government. It is no fluke that Jefferson stated this idea to a letter to a congregation of Baptists, the Baptist movement being one of the prime originators within free church Protestantism of the idea of separating the religious and political spheres.
But that doesn't explain how Jefferson's phrase and ideology ended up getting introduced into the jurisprudence interpreting and applying the First Amendment to church-state issues. That story is told over at the always interesting Volokh Conspiracy law blog. Spoiler: the KKK was involved!
Update: in response to the comment thread on this point, blogger extraordinaire and University of Wisconsin constitutional law professor Ann Althouse looks at the precise language used in the O'Donnell-Coons debate and comes to the conclusion that Coons was making a "blatant misstatement of the Establishment Clause of the First Amendment." She thinks O'Donnell's got the better of the debate on this point.
Althouse has more commentary on the debate and the terminology used by O'Donnell and Coons here. As Althouse points out, O'Donnell was making a point about the actual text of the Constitution, a point that she got right. Coons was making a point about the interpretation of that text. And how did we get to that interpretation of the text? That's what the Volokh Conspiracy post that I link to above helps us to understand.