In America, Unitarianism evolved chiefly out of the New England Congregational Churches. Yet, theological unitarianism existed among members of probably all churches. The first "officially" Unitarian Church in America -- King's Chapel, in 1786 -- was originally Anglican.
And so it is that dynamic lead to a very illuminating correspondence among James Freeman, the man who spearheaded KC's unitarian reform and America's first Episcopalian Bishops.
Bishop Seabury (aka, the infamous "Farmer" of Hamilton's Farmer Refuted) refused to ordain Freeman because of his unitarianism and thereby cut him off from the Anglican communion.
Originally it was the unitarians who were more ecumenical. They thought they could get rid of Trinitarian dogma in church liturgy and unitarians and Trinitarians could "get along" under the lowest common denominator between them. But the Trinitarians insisted the Trinity central to "real Christianity" and consequently, non-negotiable.
This entire book is worth a read. But I am going to reproduce a passage where Rev. Freeman records his meeting with Bishop Seabury, Oct. 31, 1786 [paragraph breaks added]:
My visit to Bishop Seabury terminated as I expected. Before I waited upon him, he gave out that he never would ordain me, but it was necessary to ask the question. He being in Boston last March, a committee of our church waited upon him, and requested him to ordain me, without insisting upon any other conditions than a declaration of faith in the Holy Scriptures. He replied, that, as the case was unusual, it was necessary for him to consult his presbyters, — the Episcopal clergy in Connecticut.
Accordingly, about the beginning of June, I rode to Stratford, where a convention was holding, carrying with me several letters of recommendation. I waited upon the Bishop's presbyters, and delivered my letters. They professed themselves satisfied with the testimonials which they contained of my moral character, &c, but added that they could not recommend me to the Bishop for ordination upon the terms proposed by my church.
For a man to subscribe the Scriptures, they said, was nothing; for it could never be determined from that what his creed was. Hereticks professed to believe them not less than the orthodox, and made use of them in support of their peculiar opinions. If I would subscribe to such a declaration as that I could conscientiously read the whole of the Book of Common Prayer, they would cheerfully recommend me. I answered that I could not conscientiously subscribe a declaration of that kind.
"Why not?" — "Because there are some parts of the Book of Common Prayer which I do not approve." — "What parts?" — "The prayers to the Son and the Holy Spirit." — "You do not then believe the doctrine of the Trinity?" — "No." — "This appears to us very strange. We can think of no texts which countenance your opinion. We should be glad to hear you mention some." — "It would ill become me, Gentlemen, to dispute with persons of your learning and abilities. But if you will give me leave, I will repeat two passages which appear to me decisive: There is one God, and one Mediator between God and man, the man Christ Jesus. There is but one God, the Father, and one Lord Jesus Christ. In both these passages Jesus Christ is plainly distinguished from God, and in the last, God is expressly declared to be the Father." To this they made no other reply than an "Ah!" which echoed round the room.
"But are not all the attributes of the Father," said one, "attributed to the Son in the Scriptures? Is not omnipotence for instance?" "It is true," I answered, "that our Saviour says of Himself, All power is given unto me, in Heaven and Earth. You will please to observe here that the power is said to be given. It is a derived power. It is not self-existent and unoriginated, like that of the Father." — "But is not the Son omniscient? Does he not know the hearts of men?" — "Yes, He knows them by virtue of that intelligence which he derives from the Father; but, by a like communication, did Peter know the hearts of Ananias and Sapphira."
After some more conversation of the same kind, they told me that it could not possibly be that the Christian world should have been idolaters for seventeen hundred years, as they must be according to my opinions. In answer to this, I said that whether they had been idolaters or not I would not determine, but that it was full as probable that they should be idolaters for seventeen hundred years as that they should be Roman Catholicks for twelve hundred. They then proceeded to find fault with some part of the new Liturgy.
"We observe that you have converted the absolution into a prayer. Do you mean by that to deny the power of the Priesthood to absolve the people, and that God has committed to it the power of remitting sins?" — "I meant neither to deny nor to affirm it. The absolution appeared exceptionable to some persons, for which reason it was changed into a prayer, which could be exceptionable to nobody." — "But you must be sensible, Mr. Freeman, that Christ instituted an order of priesthood, and that to them he committed the power of absolving sins. Whose soever sins ye remit they are remmitted unto him, and whose soever sins ye retain they are retained." To this I made no reply than a return of their own emphatic Ah!
Upon the whole, finding me an incorrigible heretick, they dismissed me without granting my request. They treated me, however, with great candor and politeness, begging me to go home, to read, to alter my opinions, and then to return and receive the ordination which they wished to procure me from the Bishop. I left them and proceeded to New York. When there I waited on Mr. Provost, rector of the Episcopal Church, who is elected to go to England to be consecrated a Bishop. I found him a liberal man, and that he approved of the alterations which had been made at the Chapel. Of him I hope to obtain ordination, which I am convinced he will cheerfully confer, unless prevented by the bigotry of some of his clergy.
The Episcopal ministers in New York, and in the Southern States, are not such high churchmen as those in Connecticut. The latter approach very near to Roman Catholicks, or at least equal Bishop Land and his followers. Should Provost refuse to ordain me, I shall then endeavor to effect a plan which I have long had in my head, which is to be ordained by the Congregational ministers of this town, or to preach and administer the ordinances without any ordination.
The last scheme I most approve; for I am fully convinced that he who has devoted his time to the study of divinity, and can find a congregation who are willing to hear him, is, to all intents, a minister of the gospel; and that, though imposition of hands, either of bishops or presbyters, be necessary to constitute him priest in the eye of the law, in some countries, yet that, in the eye of heaven, he has not less of the indellible character than a bishop or a patriarch. Our manly ancestors, who, however wrong they might be in some particulars, were in general sensible and judicious men, were of this opinion. One of the articles of the Cambridge platform is, That the call of the congregation only constitutes a man a minister, and that imposition of hands by bishops or elders is a mere form, which is by no means essential. The same sentiments are adopted by the most rational clergy in the present day, who give up the necessity of Ordination as indefensible, and ridicule the doctrine of the uninterrupted succession as a mere chimera. I am happy to find many of my hearers join with me in opinion upon this subject.