One reason why modern scholars don't believe Washington a Christian was because his titles for the deity didn't sound Christian. Some scholars argue his titles like "Providence," "Supreme Being," "Great Architect of the Universe" were deist. The problem with that categorization is Washington constantly referred to an intervening God. So unless we categorize Washington as a "Warm Deist" -- which term some view as an oxymoron -- that label doesn't fit. Here is how Michael Novak sees it:
He preferred what I would say are deist nouns – "the author of all the good that is, that was, and that will be;" sometimes "Father Almighty" or "the Almighty." In the back of my book, I list all the various names he gave for God – never Savior, never Redeemer, never Trinity. You had to be careful; you had Unitarians and Baptists. They didn't want to start religious arguments; they needed to pull together as one. If you look at the nouns, you can argue he was deist. But if you look at the verbs, of what he asked God to do, they only fit with the Jewish-Christian God.
In other words, his nouns for God sound deist but his verbs described an active personal God, which Novak categorizes as not a "deist" God but a "Judeo-Christian" God. I suppose an active personal God could be described as "Judeo-Christian" in a very loose sense, with a few caveats. One: Jefferson, Adams, and Franklin believed in this same active personal God. And all likely believed some of the Jewish and Christian Scriptures were legitimately revealed. Some but not all. It was the Biblical God minus all of His attributes the Bible writes about -- like wrath and jealously -- that didn't comport with their notion of "reason." Two: These Founders also believed -- rightly or wrongly -- that non-Judeo-Christian religions believed in the same active personal God they worshipped.
On the controversy over the recent prayer given by the Hindu Senate Chaplain, I've noted (and so has Ed Brayton in this great post) one of Adams' quotations speaking as though Hindus worshipped the same God that Christians did, terming the Shastra, a Hindu treatise, a work of "orthodox theology." And Washington, Jefferson, and Madison (perhaps Adams) used the term "The Great Spirit" when talking to Native Americans suggesting their pagan God was the same active personal God they worshipped.
Novak rightly points out, however, that we never see, in the names Washington used for God, "Savior," "Redeemer," or "Trinity." This utter lack of speaking in orthodox Trinitarian terms is one important reason scholars conclude Washington wasn't Christian, but "deist" or something else. And as I've noted many times, that "something else" is best described as theistic rationalism, a hybrid between strict deism and orthodox Christianity with rationalism as the trumping element.
But to be fair, one could conclude with certainty only that Washington systematically used generic philosophical titles for God all of which Jews, Christians, Muslims, Deists, and Unitarians could think aptly described "their" God.
Peter A. Lillback claimed in his book and in an article that Washington used the term "Redeemer." I've found the primary source. It was from Washington's "General Orders" that he gave to his troops, dated 11-27-1779, which reproduced verbatim one of Congress' proclamations.
Head Quarters, Moore's House, Saturday, November 27, 1779.
Parole Landaft. Countersigns Lexington, Leeds.
The Honorable the Congress has been pleased to pass the following proclamation.
Whereas it becomes us humbly to approach the throne of Almighty God, with gratitude and praise for the wonders which his goodness has wrought in conducting our fore-fathers to this western world; for his protection to them and to their posterity amid difficulties and dangers; for raising us, their children, from deep distress to be numbered among the nations of the earth; and for arming the hands of just and mighty princes in our deliverance; and especially for that he hath been pleased to grant us the enjoyment of health, and so to order the revolving seasons, that the earth hath produced her increase in abundance, blessing the labors of the husbandmen, and spreading plenty through the land; that he hath prospered our arms and those of our ally; been a shield to our troops in the hour of danger, pointed their swords to victory and led them in triumph over the bulwarks of the foe; that he hath gone with those who went out into the wilderness against the savage tribes; that he hath stayed the hand of the spoiler, and turned back his meditated destruction; that he hath prospered our commerce, and given success to those who sought the enemy on the face of the deep; and above all, that he hath diffused the glorious light of the gospel, whereby, through the merits of our gracious Redeemer, we may become the heirs of his eternal glory: therefore,
RESOLVED, That it be recommended to the several states, to appoint Thursday, the 9th of December next, to be a day of public and solemn thanksgiving to Almighty God for his mercies, and of prayer for the continuance of his favor and protection to these United States; to beseech him that he would be graciously pleased to influence our public councils, and bless them with wisdom from on high, with unanimity, firmness, and success; that he would go forth with our hosts and crown our arms with victory; that he would grant to his church the plentiful effusions of divine grace, and pour out his holy spirit on all ministers of the gospel; that he would bless and prosper the means of education, and spread the light of christian knowledge through the remotest corners of the earth; that he would smile upon the labours of his people and cause the earth to bring forth her fruits in abundance; that we may with gratitude and gladness enjoy them; that he would take into his holy protection our illustrious ally, give him victory over his enemies, and render him signally great, as the father of his people and the protector of the rights of mankind; that he would graciously be pleased to turn the hearts of our enemies, and to dispense the blessings of peace to contending nations; that he would in mercy look down upon us, pardon our sins and receive us into his favor, and finally, that he would establish the independence of these United States upon the basis of religion and virtue, and support and protect them in the enjoyment of peace, liberty and safety.33
These were not Washington's words but Congress'. These words were orthodox Christian and generally constituted an anomaly for the federal government to speak in such strong language in which non-Trinitarians and non-Christians didn't believe. The key Founders and the federal government (especially after the Constitution was ratified) far more often spoke of God, using generic philosophical titles compatible with a variety of different creeds.
That Lillback couldn't even find Washington using the term "Redeemer" but had to find it in Congress' words reproduced in Washington's "General Orders" only serves to strengthen my point -- that Washington didn't talk like an orthodox Christian.
Clearly Washington had no problem with orthodox Christian beliefs and probably didn't mind his soldiers hearing such a proclamation because such beliefs, no doubt, dominated the soldiers'/population's consciences. However, as noted, it's not at all clear that GW himself believed in orthodox Christianity. And the creedal indifference (or "latitudinarianism") of GW and the other key Founders was so strong they also had no problem with and were equally accepting of other faiths which made claims of truth incompatible with Christianity's. In other words, were Washington's soldiers predominately Muslim, the best evidence shows he'd have no problem reading them explicitly Muslim prayers.
As Ben Franklin put it, describing this creedal indifference which led him to embrace whatever "religion" the people did:
Both house and ground were vested in trustees, expressly for the use of any preacher of any religious persuasion who might desire to say something to the people at Philadelphia; the design in building not being to accommodate any particular sect, but the inhabitants in general; so that even if the Mufti of Constantinople were to send a missionary to preach Mohammedanism to us, he would find a pulpit at his service.
Evidence of GW's creedal indifference? Washington, in his writings, acted as though Protestants, Jews, Roman Catholics, and some exotic Christian Churches like Universalists (who deny eternal damnation), Quakers, and Swedenborgians were valid paths to God. He also twice, when speaking to Native Americans used the term "The Great Spirit" suggesting that pagan God was the same God he worshipped. And unlike "Allah," "The Great Spirit" doesn't even purport to be the God of Abraham, Isaac and Jacob.