David Barton is a pseudo-historian popular with some members of the Christian right because he claims that the United States was intended to be a Christian nation and the Founding Fathers, including Thomas Jefferson, were orthodox Christians who wanted Christianity to be the national religion and never intended what Jefferson celebrated as the separation of church and state.
In his latest bestseller, The Jefferson Lies: Exposing the Myths You've Always Believed About Thomas Jefferson (with a foreword by Glenn Beck), Barton attempts to show, among other things, that Jefferson, despite being identified as a Deist by legitimate historians and his biographers -- not to mention by himself! -- was nothing of the sort after all, but rather a Christian whose beliefs differed only a little from mainstream Christianity.
To be clear, while I've read enough of the book to get a handle on its assertions, I haven't gone through it cover-to-cover, so this isn't a review. Instead, I just wanted to point you to a well-informed review at Salon by Warren Throckmorton, a Christian college professor who has delved deeply into Barton's writings and discovered numerous errors and outright inventions.
It's perhaps worth noting that Dr Throckmorton is a professor of psychology rather than history, but then again, Barton's academic background consists of a degree is in Christian education from Oral Roberts University, and professional historians appear to share Throckmorton's low opinion of Barton's scholarship.