As Rodda points out, it takes some fairly awesome mental gymnastics to make a treaty that says the US is not a Christian nation mean that the US is a Christian nation.
One of the most often used arguments that the United States was not founded as a Christian nation is Article 11 of the 1797 Treaty of Peace and Friendship between the United States and the Bey and Subjects of Tripoli of Barbary. This is a pretty good argument, considering that the first sentence of that article begins with the words, "As the government of the United States of America is not in any sense founded on the Christian Religion..." Because the authors of the religious right version of American history can't deny that these words are there, they attempt to dismiss them, usually using one, or a combination of, several popular arguments.Hm ... I think this calls for another quote from The Origins of Totalitarianism
Plato, in his famous fight against the ancient Sophists discovered that their "universal art of enchanting the mind by arguments" had nothing to do with truth aimed at opinions which by their very nature are changing and which are valid only "at the time of the agreement and as long as the agreement lasts." He also discovered the very insecure position of truth in the world, for from "opinions comes persuasion and not truth."
The most striking difference between the ancient and modern sophists is that the ancients were satisfied with a passing victory of the argument at the expense of truth, where as the moderns want a more lasting victory at the expense of reality. In other words, one destroyed the dignity of human thought whereas the modern manipulators of facts stand in the way of the historian. For history itself is destroyed, and its comprehensibility ... is in danger, whenever facts are no longer held to be part and parcel of the past and present world, and are misused to prove this or that opinion.