It would be safe to say that, by the end of his life, Jefferson did not appear to subscribe to all points of the historic Christian faith—although there is evidence he did so earlier in his life. Nonetheless, he had a great interest in the teaching of Christ. As Christians, we believe that belief in Jesus involves believing in who He is and what He accomplished for sinners in His death and resurrection—not just what He taught.
Thomas Jefferson became convinced, certainly in the later phases of his life, that the doctrine of the Trinity was a false construct imposed on true “primitive” Christianity. He believed the day would come that just as the Reformation helped strip away false corruptions from the faith, so also one day the supposedly false doctrine of the Trinity would be stripped away.
In 1822, he said, “I trust that there is not a young man now living in the United States who will not die an Unitarian.” He assumed it was a given to accept that position because of two things: (1) his perception that textual proof of corruptions by scholars was growing, and (2) the number of American Christians holding Unitarian views was increasing. (By the way, this is a prediction that was not fulfilled at all because scholarship proved greater reliability of Scripture text rather than vice-versa and therefore stopped the growth of Unitarians.)
But Unitarians like Rev. Joseph Priestley and Jefferson set themselves up as the judges of what passages were corrupted and not original and what was the real words of Jesus and the apostles.
From an orthodox perspective, we can’t agree with this. The saddest thing about all this is Priestley’s definite and Jefferson’s potential rejection of the atonement. If Jesus didn’t die for our sins, then we’re in deep trouble. If that doctrine is not at the heart of the Christian message, then what is? A cross-less Christianity is no Christianity at all.
 Letter to Rev. Benjamin Waterhouse, June 26, 1822. Dickinson Adams, 405.