So their private letters often shed light on what the key FFs were really getting at. And thus far, few if any of James Wilson's have been published (as far as I know and I've looked very carefully into the record).
But we have James Wilson's vast "public works" to examine. We see Enlightenment, particularly of the Scottish kind, Locke, and natural law in there. While he doesn't cite Thomas, he sounds like him at times, and no doubt got Thomism through the Anglican and Protestant branches of the natural law.
Wilson speaks highly of "Christianity" in an abstract sense. However, it doesn't sound like "the orthodox" understanding of Christianity. Rather Wilson speaks nothing of "Grace," but substitutes "Nature" for it. Christianity becomes a generic moralizing creed whose purpose is to support the findings of "reason" and the "moral sense." After having something positive to say about Christianity, Wilson collapses it into the natural law/what's discovered by "reason" and the "senses."
It could be that Wilson was privately orthodox, that orthodox notions of "grace" really have nothing to do with public law anyway; so why should he discuss it. However I still don't think an orthodox Christian who believed the Bible the infallible Word of God would write something like the following:
III. Reason and conscience can do much; but still they stand in need of support and assistance. They are useful and excellent monitors; but, at some times, their admonitions are not sufficiently clear; at other times, they are not sufficiently powerful; at all times, their influence is not sufficiently extensive. Great and sublime truths, indeed, would appear to a few; but the world, at large, would be dark and ignorant. The mass of mankind would resemble a chaos, in which a few sparks, that would diffuse a glimmering light, would serve only to show, in a more striking manner, the thick darkness with which they are surrounded. Their weakness is strengthened, their darkness is illuminated, their influence is enlarged by that heaven-descended science, which has brought life and immortality to light. In compassion to the imperfection of our internal powers, our all-gracious Creator, Preserver, and Ruler has been pleased to discover and enforce his laws, by a revelation given to us immediately and directly from himself. This revelation is contained in the holy scriptures. The moral precepts delivered in the sacred oracles form a part of the law of nature, are of the same origin, and of the same obligation, operating universally and perpetually.
On some important subjects, those in particular, which relate to the Deity, to Providence, and to a future state, our natural knowledge is greatly improved, refined, and exalted by that which is revealed.
Got that? The purpose of revelation is to "improve,refine, and exalt," what man already knows from "reason" and "conscience" (or as he put it elsewhere, the "moral sense").
Even though Wilson doesn't mention the Trinity, what he wrote does implicitly relate to unitarianism is one important sense. People who made claims like Wilson's (that "reason" is the first revelation of God to man; scripture (or parts thereof), the second revelation supports and clarifies what man discovers first from reason) also said it was CLEAR that reason teaches 1+1+1=3 not 1 and therefore, this was one of those CLEAR discoveries of "reason" that the Bible couldn't supersede.
As Wilson put it:
These considerations show, that the scriptures support, confirm, and corroborate, but do not supercede the operations of reason and the moral sense.
Or as John Adams put it, reacting to John Disney’s thoughts:
D[isney]: The union of all Christians is anticipated, as it has been demonstrated to be the doctrine of Christ, his apostles and evangelists, as also of Moses and the prophets. Nor is it less the language of the religion of nature than of revelation . . .
A[dams]: The human understanding is the first revelation from its maker. From God; from Heaven. Can prophecies, can miracles repeal, annul or contradict that original revelation? Can God himself prove that three are one and one three? The supposition is destructive of the foundation of all human knowledge, and of all distinction between truth and falsehood. [Click the link for the primary source.]
In other words, while reason and revelation are both necessary, when nature/reason -- God's first revelation to man -- CLEARLY answers a question, you have to go with it, regardless of what you might think the Bible teaches. And on the Trinity, the answer was clear: It is, according to Adams and those who, like Wilson, followed this rationalistic method, a false doctrine.
This is why I would bet if James Wilson's private letters discussing the Trinity were found, he would deny the Trinity.
I'll keep looking for those letters.