We Unitarians, one of whom I have had the Honour to be, for more than sixty Years, do not indulge our Malignity in profane Cursing and Swearing, against you Calvinists; one of whom I know not how long you have been. You and I, once saw Calvin and Arius, on the Plafond of the Cathedral of St. John the Second in Spain roasting in the Flames of Hell. We Unitarians do not delight in thinking that Plato and Cicero, Tacitus Quintilian Plyny and even Diderot, are sweltering under the scalding drops of divine Vengeance, for all Eternity.
-- John Adams to John Quincy Adams, March 28, 1816.
The quotation indicates that, apparently, John Quincy Adams had embraced a Calvinistic form of orthodox Christianity. I'm less well read in JQA's religion; I've seen that he vacillated between unitarianism and orthodoxy for much of his adult life. I'm not sure where he ended up at death. In 1816, he seemed to be in the orthodox camp.
With that, let us observe JQA defending, to HIS father, orthodox Trinitarianism. We also see JQA defending the small c catholicism of the Christian Church. This is important. Catholicism simply means "universal." The Bible talks about Christ's "Church." As it were, the notion of a "catholic Church" is entirely biblical, and for that reason accepted by the vast majority of evangelicals/reformed Christians. They just don't believe that the Church whose Bishop of Rome is the Pope heads said Church.
All of the orthodox creeds, the Nicene, Apostles and Athanasian invoke the "catholic" church, though some translations might not use that term, but rather opt for "universal" or simply "Christian" before "church."
But do keep this (small c catholic = universal church = orthodox Trinitarian) not only reading JQA's sentiments to his father, but for the sake of context in these matters in general.
Dated, January 5, 1816:
My Dear Sir:
I plainly perceive that you are not to be converted, even by the eloquence of Massillon, to the Athanasian creed. But when you recommend to me Carlostad, and Scheffmacher, and Priestley, and Waterland, and Clerk, and Beausobre—Mercy! mercy! what can a blind man do to be saved by unitarianism, if he must read all this to understand his Bible? I went last Christmas day to Ealing Church, and heard the Reverend Colston Carr, the vicar, declare and pronounce, among other things, that whosoever doth not keep the catholic faith whole and undefiled, without doubt he shall perish everlastingly. And the catholic faith is This: That we worship one God in Trinity, and Trinity in Unity, etc.—in short the creed of Saint Athanasius; which, as you know, the eighth article of the English Church says, may be proved by most certain warrants of Holy Scripture. Now I have had many doubts about the Athanasian Creed; but if I read much more controversy about it, I shall finish by faithfully believing it. Mr. Channing says he does not believe, because he cannot comprehend it. Does he comprehend how the omnipotent, omnipresent, omniscient, infinite, eternal spirit, can be the father of a mortal man, conceived and born of a Virgin? Does he comprehend his own meaning when he speaks of God as the Father, and Christ as the Son? Does he comprehend the possibility according to human reason, of one page in the Bible from the first verse in Genesis to the last verse of the Apocalypse? If he does, I give him joy of his discovery, and wish he would impart it to his fellow Christians. If the Bible is a moral tale, there is no believing in the Trinity. But if it is the rule of faith—
I hope you will not think me in danger of perishing everlastingly, for believing too much, and when you know all, with your aversion to thinking of the Jesuits, you may think I have made a lucky escape, if I do not believe in transubstantiation. During almost the whole period of my late residence in Russia, I had the pleasure of a social and very friendly acquaintance with the Right Reverend Father in God, Thaddeus Brozowsky, then and now Father General of the Jesuits, one of the most respectable, amiable, and venerable men that I have ever known. As I was the medium of communication between him and his correspondents in the United States, he used frequently to call upon me, and I had often occasion to return his visits. We used to converse upon all sorts of topics, and among the rest upon religion. He occasionally manifested a compassionate wish for my conversion to the true Catholic faith, and one day undertook to give me a demonstration of the real presence in the Eucharist. He said it was ingeniously proved in a copperplate print which he had seen, representing Jesus Christ sitting between Luther and Calvin, each of them bearing the wafer of the communion. Each of them had also a label issuing from his lips, and, pointing with the finger to the bread, Christ was saying, "This is my body," while Luther said, "This represents my body," and Calvin, "This signifies my body." At the bottom of the whole was the question, "Which of them speaks the truth?" It was not the worthy Father's fault if I did not consider this demonstration as conclusive as he did. Another day—and it will give you an idea of the simplicity of this good man's heart—we were discussing together the celibacy of the clergy, which he deemed indispensable, that they might be altogether devoted to the service of their Lord and master, and not liable to the avocations of this world's concerns. I did not think it would be generous to remind him of the manner in which the experience of the world had shown that the vows of religious chastity usually resulted, but rather resorted to authority with regard to the principle. I observed to him that not only all the Protestant communities, but the Greek Church also, allowed the clergy to marry. Upon which, after a moment of reflection, he said, "Oui, c'est vrai. II n'y a que l'eglise romaine qui soit encore vierge!" Indeed, you must give me some credit for firmness of character, for withstanding the persuasion of such a patriarch as this.