anon,
Correct, Washington did not believe the US to be a Christian nation; he respected the separation of church and state. Personally he was an active lay Anglican/Episcopalian. He was a member of the Vestry in colonial times, which had the very important role of distributing communal poor funds. His letter to the Jewish congregation of Newport is an amazing statement of acceptance of a minority religion; one has to go all the way back to the Roman Empire to find something similar. (And as we all know, Rome did not always practice what it preached.)
Jefferson might be described as a Deist, but even that isn’t clear. He went to great lengths to disguise his actual religious views. He clearly was not a trinitarian Christian and he edited his own version of the Christian gospels, taking out everything that looked supernatural to him. Ironically, some of the worst attacks against him came during the 1800 Presidential election — his opponent John Adams was clearly a Unitarian heretic (and didn’t hide that fact).
Here is the text of Article 11 of the Treaty of Tripoli as ratified by the US Senate:
Since under the US Constitution treaties become the law of the land, this language would appear to still be in effect.
FWIW, Tripoli is now part of Libya.