Digitized Primary Sources on GW & Religion:

This little public debate between 19th Century freethinker Robert Dale Owen and conservative Christian Origen Bachelor ended up playing a fairly important role in the evolving historical understanding of George Washington's religion. Google has now digitized the book that reproduces their letters. What follows historians like Paul F. Boller have made key to their case that Washington was not a Christian in the traditional sense of the term.

The story in short is summarized from the letters which you may read here: The year is 1831 and Robert Dale Owen read a sermon from a Rev. Dr. Wilson [sic] who modern historians mistakenly think is Bird Wilson [son of Founder James Wilson, and an Episcopal minister] but is really James Renwick Willson [no relation to James Wilson, and a Calvinist covenanter Presbyterian minister], which termed all of the Presidents from Washington to Jackson "infidels" and not more than "Unitarians." Owen cites the sermon in the debate to try and score points for the "Freethinker" side. Owen tracked down Rev. Willson to confirm the contents of the controversial sermon and Willson so did. This may be the sermon or if not, it has similar contents.

JR Willson claimed to have spoken with Rev. Dr. James Abercrombie, rector at an Episcopal Church in Philadelphia Washington attended, where Abercrombie told Willson: "Sir, General Washington was a Deist." Abercrombie witnessed Washington systematically refuse to take communion, which led him to make that harsh judgment.

In the meantime Origen Bachelor, upset at the allegations against Washington sent three letters to Rev. Willson which went unanswered. Bachelor also sent a letter of inquiry to Rev. Abercrombie who confirmed that he had spoken with Willson about Washington avoiding communion, but did not remember calling him a "Deist." Though he initially didn't want Bachelor to print the letter, the letter was eventually published, the contents of which have been digitized here. Though he denies having said Washington was a "Deist," he notes because Washington refused communion he couldn't consider him a "real Christian." The following is Abercrombie's testimony:

With respect to the enquiry you make, I can only state the following facts; that as the Pastor of the Episcopal Church (an humble assistant minister to its Rector, the Rt. Rev. Dr. White) observing that on Sacrament Sundays, Gen'l Washington, immediately after the Desk and Pulpit services, went out with the greater part of the congregation, always leaving Mrs. Washington with the communicants, she invariably being one, I considered it my duty, in a sermon on Public Worship, to state the unhappy tendency of example, particularly of those in elevated stations, who invariably turned their backs upon the celebration of the Lord's Supper. I acknowledge the remark was intended for the President, and, as such, he received it. A few days after, in conversation with, I believe, a Senator of the U. S., he told me he had dined the day before with the President, who, in the course of conversation at the table, said, that on the preceding Sunday, he had received a very just reproof from the pulpit, for always leaving the church before the administration of the Sacrament; that he honored the preacher for his integrity and candour; that he had never considered the influence of his example; that he would never again give cause for the repetition of the reproof; and that, as he had never been a communicant, were he to become one then, it would be imputed to an ostentatious display of religious zeal arising altogether from his elevated station. Accordingly, he afterwards never came on the morning of Sacrament Sunday, tho', at other times, a constant attendant in the morning. Of the assertion made by Dr. Wilson in the conclusion of a paragraph of your letter, I cannot say I have not the least recollection of such a conversation, but had I made use of the expression stated, it could not have extended farther than the expression of private individual opinion. That Washington was a professing Christian is evident from his regular attendance in our church; but, Sir, I cannot consider any man as a real Christian who uniformly disregards an ordinance so solemnly enjoined by the divine Author of our holy religion, and considered as a channel of divine grace. This, Sir, is all that I think it proper to state on paper. In a conversation, more latitude being allowed, more light might, perhaps, be thrown upon it. I trust, however, Sir, you will not introduce my name in print.

I am, Sir,
Yrs.
James Abercrombie

Washington clearly wasn't a "Deist" in the Thomas Paine sense of the term, those who believe in a non-intervening God. However, Occam's Razor suggests Washington avoided communion because he didn't believe in what the act represented: Christ's Atonement. And indeed it was the Deists and Unitarians in the Trinitarian Churches who were the ones who got up, walked out and turned their backs on the Lord's Supper. John Marshall was, according to his daughter, one such Unitarian who believed in an active personal God, and understood himself to be a Christian in a broad sense, yet:

The reason why he never communed was, that he was a Unitarian in opinion, though he never joined their society. He told her he believed in the truth of the Christian Revelation, but not in the divinity of Christ; therefore he could not commune in the Episcopal Church.

Unitarians like Marshall and Washington may have considered themselves "Christian," but because they disbelieved in the Trinity, Incarnation, Atonement and Original Sin [all four of those doctrines rise and fall together] orthodox Trinitarians like Rev. Abercrombie couldn't consider them to be "real Christians," even if they weren't strict Deists.