Not only did Rev. Jeremy Belknap believe in the Sabellian heresy, he also apparently believed in universal salvation, or at the least disbelieved in eternal damnation for any soul.
What follows are some of Belknap's comments on the matter, recorded in Sprague's "Annals of the American Unitarian pulpit." The "Murray" to whom Belknap refers is John Murray a Trinitarian Universalist who helped convert Benjamin Rush to Universalism. The "Chauncy" is Rev. Charles Chauncy, one of the first and most notable unitarian universalist ministers in the Congregational Church. And a "key Patriotic Preacher."
"My practice has always been to study the Scriptures in order to find out truth and duty. What there appears sufficient evidence for I admit as truth: where the evidence is not sufficient to induce belief, I allow myself to doubt. This every man has a right to do.
"As to the controversy about Murray, I never conversed with him but once—what he said was new and strange. On examining my Bible, I saw no reason to admit it, and therefore passed it by.
"Some years ago, Murray came into my parish. Some people wished to hear him, and asked me fur the liberty of the pulpit. I said it was mine when I wanted it, and theirs when they pleased to use it. They got him to preach. I did not attend; but, understanding that he had been on the parable of the tares and wheat, I took the liberty, as I thought was my duty, to preach the next Sabbath against what I deemed the errors adopted by his followers." [Here he read the sermon.] "These were then my sentiments, and they are the same now. I never had a doubt that faith, repentance and holiness, or a change from a state of sin to newness of life, is necessary to prepare us for Heaven.
"When the Chauncy controversy came abroad, which engaged every body's attention more or less, it was natural for me to incline to one side or the other. I was inclined to call in question the immortality of the wicked in a state of future punishment, though I had no doubt of the certainty of the punishment. There are difficulties attending the subject on every side in which it can be viewed; and, after much thought upon the matter, I am inclined to this opinion;—that the revelation which God has given us in the Scriptures is intended to regulate our present conduct in this world, and to give us to understand what will be the consequences, in the future state, of our good and bad behaviour here.
"I believe the resurrection of the just and the unjust; that the life which the just shall receive from Christ at their resurrection will be immortal; and that they shall never die any more; but doubt whether it can be proved from the Scriptures that the life which the wicked shall receive at their resurrection is immortal—if it can, it will follow that their misery will never end;—but am rather inclined to think that the life which they will then receive will be a mortal life, that they will be subject to a series of misery and torment which will terminate in a second death. Whether this second death is an utter extinction of being, or whether they will be delivered from it by another resurrection, are points which I cannot determine, nor do I think the Scripture affords us full satisfaction on these subjects; so that I expect no full solution in this world, and am fully contented with believing that the surest way for us is to believe in Christ, to fear God, and work righteousness in obedience to the Gospel, and thus secure our own happiness, without prying too curiously into the secret and future designs of God. The Apostles themselves declare,—'We know but in part, and we prophecy but in part.' If the chosen and inspired ambassadors of Jesus Christ were imperfect in their knowledge, how can we expect perfection in this life?
"If, upon this declaration of my mind, you see fit to recommend to the Society to recall the invitation they have given me to settle with you, I am content."
And then there is the following from "A history of the Unitarians and the Universalists in the United States," by Joseph Henry Allen, Richard Eddy:
Rev. Dr. Jeremy Belknap, pastor of the Federal Street Congregational Church, Boston, and one of the founders of the Massachusetts Historical Society, has left an avowal of his belief in Universalism. His correspondence with Ebenezer Hazard, of Philadelphia, has been published by the Historical Society. In it Hazard acknowledges receipt of a copy of Dr. Chauncy's pamphlet in 1782, inquires who is the author, and adds: "If it is unscriptural, I am too ignorant to be able to see it. I think, however, it does honor to the mercy of the Deity, without doing injury to divine justice." Dr. Belknap replies: "The design of emitting this piece was good, but I am not altogether pleased with its execution, because it seems to be an attempt to recommend the doctrine by the force of human authority. . . . However, the truth of the case is this: the doctrine of universal restitution has long been kept as a secret among learned men. Murray has published some undeniable truths concerning it, mixed with 'a jargon of absurdity; and one Winchester among you has followed his example. . . . As to the doctrine itself, of which you desire my opinion, I frankly own to you that I have for several years been growing in my acquaintance with it and my regard for it. I wished it might be true long before I saw any just reason to conclude it was so. ... But at present I do not see how the doctrine can be disproved, if the Scripture be allowed to speak for itself, and the expressions therein used be understood in their natural sense, without any systematical or synodical comments."