I have examined all, as well as my narrow sphere, my straitened means, and my busy life would allow me ; and the result is, that the Bible is the best book in the world. It contains more of my little philosophy than all the libraries I have seen; and such parts of it as I cannot reconcile to my little philosophy, I postpone for future investigation. Priestley ought to have given us a sketch of the religion and morals of Zoroaster, of Sanchoniathon, of Confucius, and all the founders of religions before Christ, whose superiority would, from such a comparison, have appeared the more transcendent. Priestley ought to have told us that Pythagoras passed twenty years in his travels in India, in Egypt, in Chaldea, perhaps in Sodom and Gomorrah, Tyre and Sidon. He ought to have told us, that in India he conversed with the Brahmins, and read the Shasta, five thousand years old, written in the language of the sacred Sanscrit, with the elegance and sentiments of Plato. Where is to be found theology more orthodox, or philosophy more profound, than in the introduction to the Shasta? "God is one, creator of all, universal sphere, without beginning, without end. God governs all the creation by a general providence, resulting from his eternal designs. Search not the essence and the nature of the Eternal, who is one; your research will be vain and presumptuous. It is enough, that, day by day and night by night, you adore his power, his wisdom, and his goodness, in his works. The Eternal willed, in the fulness of time, to communicate of his essence and of his splendor, to beings capable of perceiving it. They as yet existed not. The Eternal willed, and they were. He created Birma, Vitsnow, and Sib." These doctrines, sublime, if ever there were any sublime, Pythagoras learned in India, and taught them to Zaleucus and his other disciples. He there learned also his metempsychosis; but this never was popular, never made much progress in Greece or Italy, or any other country besides India and Tartary, the region of the grand immortal Lama. And how does this differ from the possessions of demons in Greece and Rome, from the demon of Socrates, from the worship of cows and crocodiles in Egypt and elsewhere? After migrating through various animals, from elephants to serpents, according to their behavior, souls that, at last, behaved well, became men and women, and then, if they were good, they went to Heaven. All ended in Heaven, if they became virtuous. Who can wonder at the widow of Malabar ? Where is the lady who, if her faith were without doubt that she should go to Heaven with her husband on the one hand, or migrate into a toad or a wasp on the other, would not lie down on the pile, and set fire to the fuel ? Modifications and disguises of the metempsychosis had crept into Egypt, and Greece, and Rome, and other countries. Have you read Farmer on the demons and possessions of the New Testament? [bold face mine]As anyone can see, this quote throws a wrench into Jon's narrative about the founding and a generic god as God of the DOI. Adams says the religion of Christ is superior to all the others. Moving to the other part in bold, Adams quotes the intro to the Shasta? I've never read it, but it's clear Adams is saying this part is just as orthodox as the Bible, which it is, especially if you are a unitarian like Adams. This quote by itself doesn't have anything to do with the equality of Christianity, as he's already said Christ is superior.
As Bill Fortenberry says, "Now, you said that Adams does the "SAME THING" with the Hymn to Zeus that he did to the Hindu Shastra, and I agree. Adams recognized in the Hymn to Zeus a certain agreement with Christian doctrine, and in the Hindu Shastra, he also found certain agreements with Christianity. In the former he discovered an agreement in regards to the devotion that man owes to God, and in the latter he saw agreement in the concept that God is both one and three at the same time." All is well until that last part.
Bill gives the correct context for the below quote:
It has pleased the Providence of the first Cause, the Universal Cause, that Abraham should give religion not only to Hebrews but to Christians and Mahomitans, the greatest part of the modern civilized world.– John Adams to M.M. Noah, July 31, 1818.
In regards to Adams' letter to M. M. Noah, the comment which you quoted is nothing more than a statement of fact. All three of those religions trace their origin to Abraham. The Christians trace their religion through Christ, the Jewish through Isaac and the Muslims through Ishmael. There is much in the Bible about God's blessing on the nations that are now predominantly Muslim beginning with God's promise to bless Ishmael at Abraham's request in Genesis 17:20. Of course, this does not mean that the Muslim religion is correct, but it does show that the statement made by Adams is accurate.The Christian nation thesis is again, above reproach to any secular darts thrown its way.