With the exception of Isaac Newton, Jefferson believed that Jesus was the greatest man in history. This is why he thought that the King James Bible was not a sufficient document to show the story of his life. He thought Jesus, who was not divine, produced the greatest progress in ethics in world history, even more so than Socrates. Of all Jefferson's accomplishments, he believed that his greatest was the Virginia Statute for Religious Freedom. This statute was the first of its kind in the world because it established the separation of church and state. So it may very well be his greatest achievement.
In his time Jefferson was widely thought of as a religious radical. At one point in his life, he predicted that, in 50 years, most young men in the nation would be Unitarians. I think he might be disappointed with our current makeup. As an ex-Unitarian myself, I find this amusing. During the election of 1796, it was believed that he was going to confiscate people’s Bibles. Perhaps one of the reasons why Adams defeated him. These types of stories always remind of the 2008 election when stories of Obama being a Muslim surfaced. Of course, these were untrue. Jeffferson believed in the neutrality of the government. He didn't care what people believed, he just thought they should have the freedom to pursue their own beliefs. This was indeed very radical at the time, even now in some circles.