William Sprague's "Annals of the American Unitarian Pulpit" reproduces Rev. Belknap's argument for Sabellianism. Basically, he thought orthodox trinitarianism was to close to tritheism. As he noted:
"According to Dr. Watts' view of the present subject,— 'The Father, the Word and the Holy Ghost, are the one living and true God.' To this proposition I give my ready assent. And whoever does so, whatever be his peculiar mode of explication, I will maintain has as just a claim to the character of Orthodox as they who do it in the Athanasian sense. And for any who adopt that or any other mode of explication, to monopolize Orthodoxy to themselves, is a degree of presumption unbecoming fallible creatures, especially those who allow that the Mode of subsistence in the Sacred Three is not ascertained in Scripture; and indeed it is inconsistent with the avowed Catholicism of the ablest and best writers, who are most partial to the general Calvinistic system.
"With respect to the idea of Personality, as applicable to the Father, Son and Spirit. Dr. Watts differed from many Trinitarians, as he denied (and I think with sufficient reason) that there are in Deity three distinct Infinite Spirits, or really distinct persons, in the common sense of that term, each having a distinct intelligence, volition, power, &c., thinking such a supposition inconsistent with the proper Unity of the Godhead; which is doubtless one of the most obvious and fundamental doctrines of revelation.
"But it is to be remembered that, with regard to the definition of personality, Trinitarians widely differ among themselves. While some suppose it to be Real, others think it only Modal or nominal; and others somewhat between both. Some of the two latter classes have charged the former with Tritheism; and to me it seems difficult to clear the doctrine from the imputation. Nor can I conceive what Tritheism is, if this hypothesis does not come under the description. To assert a mere Unity of Essence or Nature will not obviate the difficulty; for three Divine persons or beings, though of the same nature, or in other words, all of them Exactly Alike, (which seem-, to be the meaning of the term and is the popular idea,) would be, as really three Gods, as three human persons of the same nature, were they in all respects alike, would be three men. Such a sentiment, I think, ought to be zealously opposed as heretical.
— "As to those who use the common Trinitarian language in the Sabellian sense, (which, upon a close inquiry, I have found to be the case with some, and have reason to think it so with many,) they have little reason to cry out 'heresy' at the mode of interpretation for which I am here apologizing.
"That it should by any be stigmatized with the name either of Socinianism or Arianism, appears to me perfectly uncandid and unjust. The Ante-Nicene fathers adopted this hypothesis. And, if I understand the great Reformer Calvin aright, he, in like manner, conceived of the Word and Spirit of God, as the Wisdom and Power, of Deity personified. The pious Mr. Baxter adopted a like personification, and severely reproves those orthodox men, who anathematize them that espouse such a mode of explaining the Trinity. Certain it is that Socinians reject such kind of language, and disavow the notion of a Trinity in any form; not now to say any thing of the atonement, which they universally deny, but which those I am defending as strenuously maintain.
"As to Arians, properly so called, if I have any idea of their sentiments, they consider the Logos and the Holy Spirit as Created Beings; which I think with Dr. Watts, is an error, most manifestly repugnant to Scripture doctrine.
"It is true Dr. Watts maintained the man Christ Jesus to have been a created being. But if, on that account, his followers are justly charged with heresy, I know not who will be exempt; for I suppose all will allow that Christ was properly Man, and as such created. Some indeed maintain that he was a human Person as really as any other man is so, and on this ground deny that his Divinity was a Real Person, distinct from that of the Father, (for otherwise there would be two persons in Christ,) while others strangely and arbitrarily suppose (to avoid this last absurdity) that the manhood of Christ was merely a created Nature. But both allow the Deity of Christ to consist in the union of the Godhead and the manhood in the person of Emanuel, so that in Him God was manifest in the flesh.' This general argument I look upon as all that is essential to true orthodoxy, and a sufficient bond of union. How much farther Christian charity may safely extend, it is not my present business to inquire."