I'm going to get a lot of grief from many of my close friends for this post, but here we go:
Thomas Jefferson was, without a doubt, one of the greatest men that this country has ever produced, and his ideas on religion in particular are so refreshing that one can hardly find anything to compare them to in this dark and dismal age.
Why should such a statement prove controversial? Many of my friends can't ? and don't ? separate Jefferson's ideas on liberty, religion, freedom of speech, republican government, etc., from his views on slavery and African Americans. By our standards, the man was abominably racist. But, by the standards of his age, he was an advanced and liberal thinker, even on the subject of race. While I in no way concur with his ideas on race, I also feel very strongly that we shouldn't throw out the baby of his immense body of radical and socially-libertarian thinking with the bathwater of his racial beliefs. There, I've said it, let's move on.
Jefferson and religion.
Jefferson was what today we'd perhaps call a liberal religionist. He was probably a Deist, believing in one God, but a God who took no active part in the affairs of this world after the act of creation. A God who has been likened to a blind clockmaker, setting the world in motion, and then letting it go as it would from there on out. He was certainly Unitarian in bent, and affirmed repeatedly the superiority, in his view, of the Unitarian faith. As such, he averred the unity of the Godhead ? no agonized Trinitarian apologetics for him ? and that Jesus wasn't God, but was fully human, albeit the greatest moral and ethical teacher the world had ever known. He also believed very strongly that one's religion was shown in one's life, in the actions that one chose to undertake in relationship to oneself and others.
A library could be filled with books on Jefferson and his life and thought. Many of those books would deal with his liberal religious beliefs. But, one book that should stand at the center of all other work in this area would be what is commonly known as The Jefferson Bible which he compiled under the working title of The Life and Morals of Jesus of Nazareth. As Jefferson himself wrote, a "more beautiful or precious morsel of ethics I have never seen." In another letter, this one to Benjamin Rush, Jefferson wrote:
To the corruptions of christianity I am indeed opposed; but not to the genuine precepts of Jesus himself. I am a Christian in the only sense in which he wished anyone to be; sincerely attached to his doctrines, in preference to all others; ascribing to himself every human excellence, and believing he never claimed any other.
Jefferson's Bible was compiled in 1803. It consists of excerpts carefully pulled from the four gospels and arranged into a chronological narrative of Jesus' life and ministry. Jefferson painstakingly removed all miracles and other things that smack of the supernatural. Make no bones about it: this is a radical, rationalistic revision of the life and message of Jesus Christ intended to portray him as a great religious and moral teacher, that and nothing more.
The book, when finished, consisted of passages from the King James Bible alongside the same text from French, Latin, and Greek bibles. Jefferson, however, didn't publish it in his lifetime; he was far too private a person for that. The book didn't see the light of day until the early twentieth century when it was reprinted for members of Congress.
This little Dover book is handsomely produced, and includes some brief background and supplementary materials on Jefferson's lower-case christianity (the book is also available in a nice hardcover from Beacon Press, the Unitarian Universalist publisher). For more on this, and other subjects, including slavery and relations with Native Americans, see The Quotable Jefferson, a thematically-organized selection of quotations, or The Viking Portable Thomas Jefferson. I prefer the latter as it includes the majority of Jefferson's public papers as well as a big chunk of letters. The problem with it is that it doesn't index by subject, so it's pretty much a hunt and peck search for anything that you may be interested in. My choice of the one-volume biographies would be Noble E. Cunningham's In Pursuit of Reason.