You can read the following historical account in the book: In Virginia, in the 1780s, Enlightenment unitarians Jefferson and Madison teamed with evangelical baptists to separate Church & State, arguing religious establishments violated the rights of conscience. In 1780 Massachusetts where secret unitarianism was brewing, their state constitution held the standing order of "Protestant Christian" Congregational Churches could be supported with government aid without violating the "rights of conscience." Eventually, "Protestant Christians" of the unitarian bent openly preaching their doctrines got their hands on establishment aid (and the Dedham decision held by law that "unitarianism" was "Protestant Christianity" and consequently eligible for such aid) and the orthodox shrieked that this fake Christian form of "infidelity" was now the "established religion" of Massachusetts. And surprise surprise they now came to understand that evangelicals Isaac Backus and John Leland were right that religious establishments really did violate the "rights of conscience." THAT is what ended Massachusetts' religious establishment in 1833, the last state to disestablish.
Some brief highlights: On page 283, the author claims unitarianism as a form of infidelity that differs almost not at all with deism, but confusingly claims the "label" Christian and that the Bible teaches its principles:
UNITARIANISM AND INFIDELITY.
Infidels, who have renounced the Christian religion, have established a system of their own, which they call Natural religion. Creation is their Bible, and they insist that the principles they embrace are everywhere to be read upon the fair face of nature. Many persons will perhaps be surprised, on being informed that this system is, in all essential points, the same with that which is avowed and defended by Unitarians. The only difference is, the Infidel acknowledges that the Bible teaches a faith totally different from that which he receives; while the Unitarian declares that this same system is that which the Bible teaches. The Unitarians of Massachusetts, and Paine, Hume, Gibbon, &c., "harmonize almost entirely in their religious sentiments. The only question between them is, whether the Bible exhibits those views of religion, which they mutually entertain." I do not here assert, that Unitarians agree with Infidels in discarding the Bible, but that the same truths which Unitarians profess to learn from the Bible, Infidels avow and defend. Paine, in his "Age of Reason," gives us his religious belief. The subjoined extracts from that notorious publication authorize the above remarks.
Such is the religious faith of Paine. He believes in the existence of God; in the perfection of his moral and natural attributes; that religion consists in imitating him; and that there is a future state of accountability. Now is not this the same system, which Unitarians insist that Jesus Christ and the apostles taught? We would not only remark, that Unitarians believe all this; but does it not comprise the fundamental principles of their faith? Does not this creed embrace everything which they deem essential in the instructions of Christ? Would not a sober person, declaring this to be his faith, be admitted to any Unitarian church? Thus do both parties believe the same system of doctrines, and the only question between them is, Do Jesus Christ and the apostles teach it? I appeal to any Unitarian, candid or uncandid, whether Unitarianism and this pure Deism of Tom Paine is not essentially the same thing? Such an one, to be consistent, should say to Paine, "My friend, you are right; but then you ought not to abuse the writers of the Bible, for they agree with you entirely. If you will examine the Bible more critically and rationally, you will perceive that yours is that pure and holy faith which the Scriptures inculcate."
Unitarians discard those peculiar doctrines which are usually regarded as the essential principles of Christianity. Paine renounces these also; and he renounces the Bible for teaching them. He thus agrees with Unitarians, not only in what they believe, but in what they do not believe.
1. The Trinity. "The ambiguous idea of a man God; the corporeal idea of the death of a God ; the mythological idea of a family of Gods; and the Christian system of Arithmetic, that three are one, and one is three, are all irreconcilable, not only to the Divine gift of reason that God hath given to man, but to the knowledge that man gains of the power and wisdom of God."
2. Divinity of Christ. "The Scriptures represent this virtuous and amiable man, Jesus Christ, to beat once both God and Man."
"As to the Christian system of faith, it appears to me as a system of Atheism; a sort of religious denial of God. It professes to believe in a man, rather than in God."
3. Atonement. "Is the gloomy pride of man become so intolerable, that nothing can flatter it, but the sacrifice of the Creator?"
The writings of the apostles "are chiefly controversial; and the subject they dwell upon, that of a man dying in agony on a cross, is better suited to the gloomy genius of a monk in a cell, than to any man breathing the open air of creation."
"It is an outrage offered to the moral justice of God, by supposing him to make the innocent suffer for the guilty, and also for the loose morality, and low contrivance of supposing him to change himself into the shape of a man, in order to make an excuse to himself for not executing his supposed sentence upon Adam."
I thought "that God was too good to do such an action, and also too Almighty to be under the necessity of doing it."
If it were not known that these extracts were from "Paine's Age of Reason," every one would suppose that they were taken from some Unitarian sermon or periodical. There certainly is a strong family resemblance.
Surely, the Infidel and the Unitarian are brought into very close alliance. They believe the same doctrines. They discard the same. The chief labor of Unitarians now seems to be, to advocate the religious system of Paine, and endeavor to prove that it is taught by Jesus and his apostles.*
And on page 274 one author rails on Joseph Priestley and shows how Trinitarianism and Unitarianism are theologically irreconcilable as they worship different gods:
Mr. M. insists that the "Trinitarian, who believes that Christ was [is] God," can with propriety "go to the communion table with a Unitarian, who believes him to have been an inferior, created, dependant being." He may not be aware, perhaps, that he is at points on this subject, not only with Trinitarians, but with the most respectable Unitarians. "I do not wonder," says Dr. Priestley, "that yon Calvinists entertain and express a strongly unfavorable opinion of us Unitarians. The truth is, there neither can nor ought to be any compromise between us. If you are right, we are not Christians at all; and if we are right, you are gross idolators." "Opinions such as these," says Mr. Belsham, "can no more harmonize with each other, than light and darkness, than Christ and Belial. They who hold doctrines so diametrically opposite cannot be fellow-worshippers in the same temple."—Does our author believe that the primitive disciples would have gone to the Lord's Table with professed idolators? Yet some American Unitarians have not hesitated to say, (with Dr. Priestley, as above quoted,) that those who worship the Lord Jesus Christ are idolaters.
No doubt these writings are "loaded" towards the evangelical-Trinitarian perspective. Many unitarians argued they were NOT with the Deists and were Bible believing Christians. Further, after studying the writings of Priestley et al., while they did sometimes claim that Trinitarianism was idol worship, elsewhere they stated, more or less, as long as Trinitarians learned to downplay that doctrine, they COULD worship at the same table together because both worshipped God the Father. In the end most orthodox Trinitarians proved to be FAR less accepting of the Unitarians than vice versa. But then again that's just "spiritual discernment," something many orthodox pride themselves in possessing in abundance and something unitarians went out of their way towards which to be indifferent.