David Barton is a "historical reclamationist", which means he takes things from our nation's history and "reclaims" them; in other words, changes them around to his liking. He recently wrote a book called The Jefferson Lies: Exposing The Myths You've Always Believed About Thomas Jefferson, and went on The Daily Show With Jon Stewart to promote it. What resulted was a very interesting discussion, not only about Jefferson and his beliefs, but about Barton's claims and the fact that he is not a "historian" in the technical sense of the word.
Barton said in the interview that one of the biggest misconceptions about Jefferson is that he was an atheist--although some who are versed on the topic say they don't recall that ever being an issue--and the entire point of his book seems to be geared towards proving Jefferson wasn't an atheist, but rather approved of the mixture of religion and public life.
The first part of the interview deals with this subject, but Stewart steers the conversation towards religion and the integration/separation of it and state. As he talks, Barton gets more and more political, which causes some to believe political motivation is the entire reason he does what he does, rather than a genuine interest in history. Stewart calls Barton out several times for going back on previous statements and making false claims, which Barton brushes off.
The interview is quite long--about 40 minutes--but if you have the time, watch the entire thing.