I've spent much time researching John Adams' unitarianism, which could be quite militant at times. I've spent less time on his son, John Quincy Adams' creed. I know at one time JQA was a Calvinist/Trinitarian and had some very interesting discussions and debates with his father in their exchange of letters.
However, the younger Adams apparently converted to something like unitarianism as he aged.
Below is what Koty Arnold has written and compiled:
... The letter where JQ "defends" the Trinity to his father is somewhat lukewarm, though he at that time did appear to adhere to the doctrine. ... [I]n the span of a few years, JQA would conform to the Unitarian liberalism that was then so popular in New England. He wrote in his memoirs about his contempt for orthodox Christianity, especially Calvinism, for its belief in doctrines like the atonement and original sin. The Trinity he dismissed as "unimportant" to the Christian religion, which is really just about earthly moral conduct."Solemn nonsense and inconceivable absurdity. This is the impression which I can never remove from my mind when I hear a Calvinistic preacher hammering upon that everlasting anvil of the atonement. "Incredulous odi"--I disbelieve and I hate. It is always to me an admonition of the weakness of the human intellect. That the execution, as a malefactor, of one person, the Creator of all worlds, eighteen hundred years ago, should have redeemed me, born nearly eighteen centuries after his death, from eternal damnation is not only too shocking for my belief, but I ask myself what there can be above the level of the beasts which perish in the animated being that can believe it. A melancholy monument of mental aberration and impotence.""That man is a vicious, wicked animal is the fundamental doctrine of the Christian religion. That he cannot save himself from eternal punishment is the doctrine of the Catholic churches, and of Calvin. If he cannot save himself, he is not a responsible being; that is the conclusion of justice and a conclusion from which I could not escape if I would. The mission of Christ was to teach all mankind the way to salvation. His death, an ignominious death, was necessary to the universal spread of His doctrine. He died for mankind, as Curtius died for his country, as Codrus died for his people. In this sense I can believe the doctrine of the atonement, and in no other. Christ died as a man, not as God.""The only Importance of religion to my mind consists in its influence upon conduct; and upon the conduct of mankind the question of Trinity or Unity, or of the single or double personal nature of Christ has or ought to have no bearing whatsoever.""I told him in substance what I have written to my son George, that I believed the nature of Jesus Christ was superhuman; but whether he was God or only the first of created beings was not clearly revealed to me in the Scriptures."
Read more about Mr. Arnold here.
Update: As Tom Van Dyke points out, JQA was arguably agnostic on the Trinity during this time.
"I did not prescribe to many of his doctrines, particularly not to the fundamental one of his Unitarian creed. I believe in one God but his nature is incomprehensible to me; and of the question between the Unitarians and the Trinitarians, I have no precise belief, because I have no definite understanding."