More clues on George Washington's belief in religious orthodoxy (or lack thereof): From his letter to ANNIS BOUDINOT STOCKTON, August 31, 1788:
"But, with Cicero in speaking respecting his belief of the immortality of the Soul, I will say, if I am in a grateful delusion, it is an innocent one, and I am willing to remain under its influence."
On the context of the passage, (check it for yourself, that was an excerpt and the entire thing is linked above) that line was an aside inserted into a letter on a different subject where he defends stoicism over epicureanism. Also, his prose here seems a bit "flirtatious" (GW was known to be a flirt with the ladies) and the subject of "sex" (i.e. "gender") often comes up.
But, on what he actually says about the afterlife, it seems not to fit with the notion that Washington was a Christian. First, orthodox Christians in general and his Anglican/Episcopal Church in particular have formed definite opinions on the matter. It would seem that were he a Christian/doctrinaire Anglican, he would cite his Church or the Bible as authority on the matter. But he doesn't. He says he believes in the Immortality of the soul after Cicero, a figure from the Pagan Ancient Rome. Indeed, out of all of the key Founders, Washington seemed the most enamored with pagan classical antiquity; as alluded to above, he adhered to a "Stoic" philosophy, which has its roots in Greco-Romanism. His favorite character from antiquity was the Roman Senator, Cato the Younger (a man who did a very un-Christian thing -- commit suicide as a matter of principle) and had Addison's play enacted for his soldiers in Valley Forge to inspire them.
Finally, I've also read thoughts from Adams and other key Founders where they suggest, though they believe in the afterlife, there is a possibility that they are wrong; but if such is a "delusion," it is a harmless one, and one that makes life easier to take.