Again, my doubts are not that Washington was or was not a Christian. I could see either (though, having taken an extensive look, I am left where most scholars are: we don’t really know). What I am highly dubious of is that Washington would deviate from his well-known quietude on religious matters. While he would not speak openly of his beliefs, he made it plain that he considered it inappropriate to speak about them, especially in office (he even famously misread a Continental Congress decree of fasting and prayer to omit a reference to Christ).
What makes me doubt, for instance, the claim that Washington prayed at Valley Forge in the way the famous painting depicted is not that I don’t think he ever could have prayed. It is because he never would have prayed as is described: out loud, kneeling in public like a Pharisee for our special vistor to creep up and observe like a boy in a Mark Twain tale. What your cite doesn’t think to mention is that story originally came not from Potts directly, but through the intercession of the redoubtable Parson Weems, famous for the cherry tree story. THAT is why it isn’t taken very seriously by historians. You can read more in Rupert Hughes’ George Washington (1930) vol. III, ch. 25
You also seem not understand what Diesm is. It is a form of theism, not atheism. It includes Providence a Creator, and so on. It simply does not include divinity of Christ or doctrines like the Trinity or original sin, salvation, etc…
So, I’m not sure why you think, for instance, a reference to Abraham makes Washington out to be a believer in the divinity of Christ. Or even that he instructs Indians to be schooled as Christians (Christianizing was seen as the best way to civilize Indians).
Further, your first cite says: “Washington’s own contemporaries did not question his Christianity but were thoroughly convinced of his devout faith.”
This is simply not true. In fact, it’s patently ridiculous, because Spark’s book, quoted extensively on that page, was in part prompted by so many people doubting it! The fact is, many contemporary Christians fretted over Washington’s beliefs, and tried again and again to get him to say something direct about his religion. Indeed, it made for some rather amusing escapades:
“Dr. Rush told me (he had it from Asa Green) that when the clergy addressed General Washington, on his departure from the government, it was observed in their consultation that he had never, on any occasion, said a word to the public which showed a belief in the Christian religion, and they thought they should so pen their address as to force him at length to disclose publicly whether he was a Christian or not. However, he observed, the old fox was too cunning for them. He answered every article of their address particularly, except that, which he passed over without notice.” Thomas Jefferson, Jefferson’s Works (Vol. 4) p. 572
Even Washington’s friends and the religious leaders at Washington’s church (as I have already noted, the rector at Washingon’s church thought he was a Deist) questioned what he believed. He coyly avoided all direct inquiry. Jefferson said he wasn’t sure, and that Morris told him that Washington had no more belief in Christianity than Morris did.
So, utterly regardless of whether Washington was a Christian or not, this cite starts off with an easily demonstrable lie: a lie that it provides internal evidence for debunking (containing documents which are responses to the then contemporary suggestion that Washington might not have been a Christian. What we know for sure is that Martha Washington was a Christian. What we know is that Washington had great respect for Christianity (though, far from being as described by Nelly, who knew him only briefly, he slipped out the back of his church to avoid communion, and when his pastor told him that this was disruptive, Washington was apologetic and simply ceased to come on the days when communion was given. We know that he shared the belief with many Deists that Christianity was a great device for enriching, civilizing, and pacifying the common man and “savages.” But none of this tells us much about Washington’s private beliefs.
—We see the same type of “evidence” which made Plymoth Rock a national shrine. On the basis of one man’s word a monument was created, but it has been roundly debunked by historians.—
A ninety-year old man recalling it thirdhand: his father, who had arrived three years AFTER the Mayflower, told him about it when he was a boy.
Plymouth Rock is actually even more hilarious than that. You see, sometime after it gained small fame, the people of Plymoth realized that the rock was now much too far inland to make much of a visual impact or reasonable first landing site. So they tried to move it closer to the ocean. In the process, they broke it in half!
And of course, the Pilgrims didn’t land first in Plymouth anyway: they first landed in Provincetown.
Don’t even get me started on the Liberty Bell. 
