I cannot more strongly recommend reading the entire correspondence between John Adams and Thomas Jefferson. There you will learn what both of these men really thought on religion. And surprisingly, Adams and Jefferson were almost entirely agreed on the basics and only argued over the finer details of religion. (As I've said before, it's likely that Washington, Franklin, Madison and other key Founders believed the same).
These Founders were fascinated by the theories of their friend, English Unitarian Joseph Priestly (also the discoverer of oxygen). Priestly argued that Christianity had been corrupted (He wrote a book entitled History of the Corruptions of Christianity). And the "corruptions" to which he referred were the central tenets of both Catholic Dogma and orthodox Trinitarian Protestant Christianity (most notably the Trinity and the Athanasian Creed).
Adams was more of a critic of Priestly than was Jefferson; but not because Adams disagreed with Priestly's rejection of orthodox Christianity (Adams clearly rejected those doctrines as well). Rather, Adams nitpicks on a lot if the finer details of Priestly's beliefs (which could be quite strange). Adams agreed with Priestly that Christianity had been corrupted. Not just the dogmas and doctrines, but also the Bible itself. This is important to note: it's very easy to quote Adams out of context in order to try to claim him as an orthodox Christian. Adams did say things in his writings that are easily misconstrued. For instance, in his letter to Jefferson dated December 25, 1813, "I have examined all, as well as my narrow Sphere, my streightened means and my busy Life would allow me; and the result is, that the Bible is the best book in the World." Now there, he does sound like an orthodox Christian. But one must read the entire letter in context. And then we see it reveals something entirely different.
He is discussing Priestly's work and prefaces his statement to Jefferson by noting that Man's Reason is the Ultimate discerner of the Truth and that it takes precedence over Revelation; indeed it makes Revelation entirely unnecessary:
Priestly ought to have done impartial justice to Phylosophy and Phylosophers, Phylosophy which is the result of Reason, is the first, the original Revelation of The Creator to his Creature, Man. When this Revelation is clear and certain, by Intuition or necessary Induction, no subsequent Revelation supported by Prophecies or Miracles can supercede it.
Adams further believed that the Truth is to be found in all religions (hence, he was a universalist). And that all religions, like Christianity, were universally corrupted by dogma. As badly corrupted as Christianity was, it had still been less corrupted than all of the other religions. Adams resolutely used the word "Christian" and "Christianity" in a positive sense. However, Adams, like Jefferson and Priestly, wants to claim the word "Christian" away from the orthodox clerics and use it to describe his very unorthodox, Enlightenment-influenced, rationalist, Unitarian-universalist personal set of beliefs.
When Adams "finds" the Truth in other religions that parallel teachings found in Christianity, he goes so far as to call these Eastern Pagan religions "Christian." To Adams, one was a true Christian if one was Christlike, even if one had nothing to do with the Christian tradition. Were he alive, I'm convinced that Adams would have called Ghandi, "Christian." (And keep in mind Calvin and the Popes were not "true Christians"). Here (in the same letter) is Adams describing Hinduism and noting how it contains the same basic teachings as Christianity:
Where is to be found Theology more orthodox or Phylosophy more profound than in the Introduction to the Shast[r]a [a Hindu Treatise]? “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."
Here is Adams, in his letter to Jefferson dated Oct. 4, 1813, finding "Christianity" in Pagan Greek philosophy:
θέμίς was the Goddess of honesty, Justice, Decency, and right; the Wife of Jove, another name for Juno. She presided over all oracles, deliberations and Counsells. She commanded all Mortals to pray to Jupiter, for all lawful Benefits and Blessings.
Now, is not this, (so far forth) the Essence of Christian devotion? Is not this Christian Piety? Is it not an Acknonowledgement [sic] of the existence of a Supream Being? of his universal Providence? of a righteous Administration of the Government of the Universe? And what can Jews, Christians, or Mahometans do more?
Moses says, Genesis. I. 27. ["]God created man in his own image." What then is the difference between Cleanthes and Moses? Are not the Being and Attributes of the Supream Being: The Resemblance, the Image the Shadow of God in the Intelligence, and the moral qualities of Man, and the Lawfulness and duty of Prayer, as clear[l]y asserted by Cleanthes as by Moses? And did not the Chaldeans, the Egyptians the Persians the Indians, the Chinese, believe all this, as well as the Jews and Greeks?...I believe Cleanthes to be as good a Christian as Priestley.
As I said above, although Adams revered the Bible, he clearly thought that it had been corrupted and was thus errant. It's just the Bible, as a book of Revelation, contained more of his "little Phylosophy" than any other book from other Worldly religions, all of which at once (like the Bible) contained the "Truth" but were also "corrupted" by dogma.
So, according to Adams, what the philosophically minded man should do, using his Reason as the ultimate guide, was look to all of the World's religions and philosophies and separate the wheat from the chaff or, as Jefferson put it (while describing the way he reads the Bible), try to find the "diamonds" of Truth in the "dunghill" of religious dogma.
And in fact, just as Jefferson cut out entire passages of the Bible which could not be confirmed by Reason and were thus "corrupt," Adams (approving of Jefferson's work) had his own idea to do the same. Adams stated in his letter dated Nov. 14, 1813, "I admire your Employment, in selecting the Philosophy and Divinity of Jesus and seperating it from all intermixtures. If I had Eyes and Nerves, I would go through both Testaments and mark all that I understand."
Adams in that letter doubts that the Ten Commandments were the right version. He asks Jefferson:
Among all your researches in Hebrew History and Controversy have you ever met a book, the design of which is to prove that the ten Commandments, as We have them in our Catechisms and hung up in our Churches, were not the Ten Commandments written by the Finger of God upon tables, delivered to Moses on mount Sinai and broken by him in a passion with Aaron for his golden calf, nor those afterwards engraved by him on Tables of Stone; but a very different Sett of Commandments?
There is such a book by J.W. Goethens Schristen. Berlin 1775-1779. I wish to see this Book.
Adams then goes on to recite from Exodus what he thinks may be the true Ten Commandments. He then notes:
When and where originated our Ten Commandments? The Tables and The Ark were lost. Authentic copies, in few, if any hands; the ten Precepts could not be observed, and were little remembered.
If the Book of Deuteronomy was compiled, during of after the Babilonian Captivity, from Traditions, the Error or Amendment might come in there.
Adams was convinced that the Bible, like all religious books, contained "errors and amendments" which had corrupted it. And that there was a conspiracy of those in clerical and political power throughout the ages to keep the ultimate entire "Truth" secret. As commonly presented, the different religions of the world contained "Problems, Conjectures, and paradoxes"; but if these destroyed secret teachings were uncovered (and in the future he had hopes that such Truth would be uncovered), questions would be answered and "corruptions" exposed.
From the same letter:
The Daemon of Hierarchical despotism has been at Work, both with the Mishna and Gemara. In 1238 a French Jew, made a discovery to the Pope (Gregory 9th) of the heresies of the Talmud. The Pope sent 35 Articles of Error, to the Archbishops of France, requiring them to seize the books of the Jews, and burn all that contained any Errors. He wrote in the same terms to the Kings of France, England Arragon, Castile Leon, Navarre and Portugal. In consequence of this Order 20 Carloads of Hebrew Books were burnt in France: and how many times 20 Cartloads were destroyed in the other Kingdoms? The Talmud of Babylon and that of Jerusalem were composed from 120 to 500 Years after the destruction of Jerusalem. If Lightfoot derived Light from what escaped from Gregorys fury in explaining many passages in the New Testament by comparing the Expressions of the Mishna, with those of the Apostles and Evangelists, how may proofs of the Corruptions of Christianity might We find in the Passages burnt?
Finally, Adams thought that the library at Alexandria held the answers to many of these questions and was destroyed precisely to keep the knowledge secret. From his October 4, 1813 letter:
On the other hand how do We know how much Moses Samuel Joshua David Solomon and Esdrass, Daniel Ezekiel, Isiah and Jeremiah learned in Babilon Egypt and Persia? The destruction of the Library at Alexandria, is all the Answer We can obtain to these Questions. I believe that Jews Grecians Romans and Christians all conspired, or connived At that Savage Catastrophy.
Religious and philosophical questions fascinated John Adams. And he was a man on a relentless search for the Truth. We can admire and learn from his desire to know. Thank you Mr. Adams.