My understanding of the history: In the 18th Century theological unitarianism was not considered real Christianity but "heresy" or "infidelity." Yet, such heresy was quite popular in educated 18th Century circles (and among America's key founders). This heresy was even being preached from the pulpit of New England's "Puritan" Congregational Churches. Eventually, in the 19th Century many of these New England Churches officially became Unitarian, and with Harvard officially adopting Unitarianism, and with the "rights of conscience" more firmly established in American law and culture, individuals felt freer to speak their mind and publicly assert what was once infidelity as a valid form of "Protestant Christianity."
Indeed, Philip Hamburger, in Separation of Church and State, notes one way in which Unitarians and Calvinists deflected their theological dispute was to come together, albeit for different reasons, to combat Roman Catholicism. It was liberal (Unitarian) and conservative (Calvinist) Protestants united against Roman Catholicism, which Hamburger argues eventually became Protestants United for Separation of Church and State (with underlying anti-Roman Catholic motives), which eventually became Americans United for Separation of Church and State.
The question, though, is what was being called liberal Protestant Christianity (Unitarianism), though it might have been "Protestant," was it truly Christian? Ironically, whatever the theological disputes between traditionally minded Catholics and Protestants, arguably their theologies were closer to one another's, than either were to "Unitarian Christianity," which rejected core doctrines of traditional Christianity.
For instance, at Sparks' ordination Channing states the following on the Trinity:
We object to the doctrine of the Trinity, that, whilst acknowledging in words, it subverts in effect, the unity of God. According to this doctrine, there are three infinite and equal persons, possessing supreme divinity, called the Father, Son, and Holy Ghost. Each of these persons, as described by theologians, has his own particular consciousness, will, and perceptions. They love each other, converse with each other, and delight in each other's society. They perform different parts in man's redemption, each having his appropriate office, and neither doing the work of the other. The Son is mediator and not the Father. The Father sends the Son, and is not himself sent; nor is he conscious, like the Son, of taking flesh. Here, then, we have three intelligent agents, possessed of different consciousness, different wills, and different perceptions, performing different acts, and sustaining different relations; and if these things do not imply and constitute three minds or beings, we are utterly at a loss to know how three minds or beings are to be formed. It is difference of properties, and acts, and consciousness, which leads us to the belief of different intelligent beings, and, if this mark fails us, our whole knowledge fall; we have no proof, that all the agents and persons in the universe are not one and the same mind. When we attempt to conceive of three Gods, we can do nothing more than represent to ourselves three agents, distinguished from each other by similar marks and peculiarities to those which separate the persons of the Trinity; and when common Christians hear these persons spoken of as conversing with each other, loving each other, and performing different acts, how can they help regarding them as different beings, different minds?
We do, then, with all earnestness, though without reproaching our brethren, protest against the irrational and unscriptural doctrine of the Trinity. "To us," as to the Apostle and the primitive Christians, "there is one God, even the Father." With Jesus, we worship the Father, as the only living and true God. We are astonished, that any man can read the New Testament, and avoid the conviction, that the Father alone is God. We hear our Saviour continually appropriating this character to the Father. We find the Father continually distinguished from Jesus by this title. "God sent his Son." "God anointed Jesus." Now, how singular and inexplicable is this phraseology, which fills the New Testament, if this title belong equally to Jesus, and if a principal object of this book is to reveal him as God, as partaking equally with the Father in supreme divinity! We challenge our opponents to adduce one passage in the New Testament, where the word God means three persons, where it is not limited to one person, and where, unless turned from its usual sense by the connexion, it does not mean the Father. Can stronger proof be given, that the doctrine of three persons in the Godhead is not a fundamental doctrine of Christianity?
So entirely do the Scriptures abstain from stating the Trinity, that when our opponents would insert it into their creeds and doxologies, they are compelled to leave the Bible, and to invent forms of words altogether unsanctioned by Scriptural phraseology. That a doctrine so strange, so liable to misapprehension, so fundamental as this is said to be, and requiring such careful exposition, should be left so undefined and unprotected, to be made out by inference, and to be hunted through distant and detached parts of Scripture, this is a difficulty, which, we think, no ingenuity can explain.
We also think, that the doctrine of the Trinity injures devotion, not only by joining to the Father other objects of worship, but by taking from the Father the supreme affection, which is his due, and transferring it to the Son.
After denying the Trinity and Incarnation, Channing then denies Christ's Atonement:
We farther agree in rejecting, as unscriptural and absurd, the explanation given by the popular system, of the manner in which Christ's death procures forgiveness for men. This system used to teach as its fundamental principle, that man, having sinned against an infinite Being, has contracted infinite guilt, and is consequently exposed to an infinite penalty. We believe, however, that this reasoning, if reasoning it may be called, which overlooks the obvious maxim, that the guilt of a being must be proportioned to his nature and powers, has fallen into disuse. Still the system teaches, that sin, of whatever degree, exposes to endless punishment, and that the whole human race, being infallibly involved by their nature in sin, owe this awful penalty to the justice of their Creator. It teaches, that this penalty cannot be remitted, in consistency with the honor of the divine law, unless a substitute be found to endure it or to suffer an equivalent. It also teaches, that, from the nature of the case, no substitute is adequate to this work, save the infinite God himself; and accordingly, God, in his second person, took on him human nature, that he might pay to his own justice the debt of punishment incurred by men, and might thus reconcile forgiveness with the claims and threatenings of his law. Such is the prevalent system. Now, to us, this doctrine seems to carry on its front strong marks of absurdity; and we maintain that Christianity ought not to be encumbered with it, unless it be laid down in the New Testament fully and expressly. We ask our adversaries, then, to point to some plain passages where it is taught. We ask for one text, in which we are told, that God took human nature that he might make an infinite satisfaction to his own justice; for one text, which tells us, that human guilt requires an infinite substitute; that Christ's sufferings owe their efficacy to their being borne by an infinite being; or that his divine nature gives infinite value to the sufferings of the human. Not ONE WORD of this description can we find in the Scriptures; not a text, which even hints at these strange doctrines. They are altogether, we believe, the fictions of theologians. Christianity is in no degree responsible for them. We are astonished at their prevalence. What can be plainer, than that God cannot, in any sense, be a sufferer, or bear a penalty in the room of his creatures? How dishonorable to him is the supposition, that his justice is now so severe, as to exact infinite punishment for the sins of frail and feeble men, and now so easy and yielding, as to accept the limited pains of Christ's human soul, as a full equivalent for the endless woes due from the world? How plain is it also, according to this doctrine, that God, instead of being plenteous in forgiveness, never forgives; for it seems absurd to speak of men as forgiven, when their whole punishment, or an equivalent to it, is borne by a substitute? A scheme more fitted to obscure the brightness of Christianity and the mercy of God, or less suited to give comfort to a guilty and troubled mind, could not, we think, be easily framed.
We believe, too, that this system is unfavorable to the character. It naturally leads men to think, that Christ came to change God's mind rather than their own; that the highest object of his mission was to avert punishment, rather than to communicate holiness; and that a large part of religion consists in disparaging good works and human virtue, for the purpose of magnifying the value of Christ's vicarious sufferings. In this way, a sense of the infinite importance and indispensable necessity of personal improvement is weakened, and high-sounding praises of Christ's cross seem often to be substituted for obedience to his precepts. For ourselves, we have not so learned Jesus. Whilst we gratefully acknowledge, that he came to rescue us from punishment, we believe, that he was sent on a still nobler errand, namely, to deliver us from sin itself, and to form us to a sublime and heavenly virtue. We regard him as a Saviour, chiefly as he is the light, physician, and guide of the dark, diseased, and wandering mind. No influence in the universe seems to us so glorious, as that over the character; and no redemption so worthy of thankfulness, as the restoration of the soul to purity. Without this, pardon, were it possible, would be of little value. Why pluck the sinner from hell, if a hell be left to burn in his own breast? Why raise him to heaven, if he remain a stranger to its sanctity and love? With these impressions, we are accustomed to value the Gospel chiefly as it abounds in effectual aids, motives, excitements to a generous and divine virtue. In this virtue, as in a common centre, we see all its doctrines, precepts, promises meet; and we believe, that faith in this religion is of no worth, and contributes nothing to salvation, any farther than as it uses these doctrines, precepts, promises, and the whole life, character, sufferings, and triumphs of Jesus, as the means of purifying the mind, of changing it into the likeness of his celestial excellence.
Unitarianism seems as far removed from traditional orthodox Christianity as is Mormonism. To tie this back to America's founders and religion, in a big tent sense that includes trinitarians, unitarians, universalists, Mormons, Jehovah's Witnesses, cafeteria Christians, rationalistic Christians influenced by deism, the religion of the key founders like Washington, Adams, Jefferson, Madison, and Franklin, could be considered "Christian." But in a narrow sense that strictly defines Christianity according to its trinitarian orthodoxy, the beliefs of the key founders as well as Channing, Sparks and the 19th Century "liberal Protestants," were not "Christian."
Sparks is also notable because he, as Washington's biographer, and relying on the testimony of Washington's step-granddaughter/adopted daughter Nelly Custis argued on behalf of GW's "Christianity." Well, what did Sparks consider qualified as "Christianity," and what was he arguing GW was not? It's entirely possible that Sparks meant Washington was neither an atheist nor a strict deist, but a "Christian" in some broader sense, indeed the way he Sparks was. Arguably these broader forms of "Christianity" are not real Christianity, just as the broader forms of "Deism" that accept an active personal God and an intervening Providence are not real Deism.