John Quincy Adams was a Unitarian.
From: Peter Roberts, "John Quincy Adams" page in "God and Country" section of "Science Resources on the Net" website (http://www.geocities.com/peterroberts.geo/Relig-Politics/JQAdams.html; viewed 29 November 2005):
Religious Affiliation: Unitarian
"At this day, religious indulgence is one of our clearest duties, because it is one of our undisputed rights. While we rejoice that the principles of genuine Christianity have so far triumphed over the prejudices of a former generation, let us fervently hope for the day when it will prove equally victorious over the malignant passions of our own." -- Oration at Plymouth, 22 December 1802
"The neglect of public worship in this city is an increasing evil, and the indifference to all religion throughout the whole country portends no good. There is in the clergy of all the Christian denominations a time-serving, cringing, subservient morality, as wide from the spirit of the Gospel as it is from the intrepid assertion and vindication of truth. The counterfeit character of of a very large portion of the Christian ministry of this country is disclosed in the dissensions growing up in all the Protestant churches on the subject of slavery. The abolitionists assume as the first principle of all their movements that slavery is sin. Their opponents, halting between the alternative of denying directly this position and of admtting the duty binding upon them to bear their own testimony against it, are prevaricating with their own consciences, and taxing their learning and ingenuity to prove that the Bible sanctions slavery; that Abraham, Isaac, and Paul were slave-holders; and that St. Paul is the apostle of man-stealers, because he sent Onesimus back to his master Philemon. These preachers of the Gospel might just as well call our extermination of the Indians an obedience to Divine commands because Jehova commanded the children of Israel to exterminate the Canaanitish nations." -- Diary, 27 May 1838