This is a really great project. Rare books previously available in college libraries in various parts of the world have been digitized on google. The collection grows daily. Unfortunately copyright laws prevent you from seeing the whole edition of newer books protected by copyright. But if one specializes in 18th and 19th Century history as I do, much of those works are public domain. When I first started researching the Founders and Religion not too long ago (5 years ago), I'd buy books from notable scholars who had to read the manuscripts themselves or earlier collections and who would excerpt portions of those particular sources. Now, I'm finding I'm able to confirm these sources through google books and in many instances, read the entire works in context not just the excerpts that scholars give.
Almost everything Washington and Jefferson wrote is archived online anyway. And there is one book by Lenni Brenner which reproduces the entire writings of everything Jefferson and Madison ever said on religion.
But with someone like John Adams, his works (as far as I know) are not all available online as are the works of other founders. James H. Hutson's quote book offers many interesting quotations illustrating Adams' religious heterodoxy. Often now I can read the entire letters Adams wrote, that Hutson excerpted, by searching for them on google books. For instance, in one quotation Adams proudly proclaims his Unitarianism to Jedidiah Morse in a letter dated May 15, 1815. I reproduced what Hutson excerpted in this post. I can now read the entire letter here, which I'll reproduce below.
"I thank you thank you for your favour of the 10th, and the pamphlet enclosed, entitled, 'American Unitarianism.' I have turned over its leaves, and found nothing that was not familiarly known to me. In the preface, Unitarianism is represented as only thirty years old in New-England. I can testify as a witness to its old age. Sixty-five years ago, my own minister, the Rev. Lemuel Bryant; Dr. Jonathan Mayhew, of the West Church in Boston; the Rev. Mr. Shute, of Hingham; the Rev. John Brown, of Cohasset; and perhaps equal to all, if not above all, the Rev. Mr. Gay, of Hingham, were Unitarians. Among the laity how many could I name, lawyers, physicians, tradesmen, farmers! But at present I will name only one, Richard Cranch, a man who had studied divinity, and Jewish and Christian antiquities, more than any clergyman now existing in New England. More than fifty years ago, I read Dr. Clarke, Emlyn, and Dr. Waterland: do you expect, my dear doctor, to teach me any thing new in favour of Athanasianism? — There is, my dear Doctor, at present existing in the world a Church Philosophick. as subtle, as learned, as hypocritical, as the Holy Roman Catholick, Apostolick, and Ecumenical Church. The Philosophical Church was originally English. Voltaire learned it from Lord Herbert, Hobbes, Morgan, Collins, Shaftsbury, Bolingbroke, &c. &c. &c. You may depend upon it, your exertions will promote the Church Philosophick, more than the Church Athanasian or Presbyterian. This and the coming age will not be ruled by inquisitions or Jesuits. The restoration of Napoleon has been caused by the resuscitation of inquisitors and Jesuits.
I am and wish to be
Quincy, May 15th, 1815.
"Athanasianism" refers to the orthodox Trinitarian Christianity, i.e., the Athanasian Creed. When Adams refers to the "Church Philosophick," he's clearly referring to strict Deism, that which could often virulently attack the Christian religion. Adams noted he has as much distaste for strict Deism as he does orthodox Christianity, and saw his "unitarianism" as a rational middle ground between the two systems. Yet, the term "infidel" during the founding era often meant someone to one's religious left. To Adams, those above mentioned Deists or atheists would be "infidels." However to orthodox Trinitarian Christians like Jedidiah Morse, Adams and his fellow theological unitarians were "infidels" to be lumped in with the Deists. Adams et al. didn't consider themselves "infidels" (I don't think the Deists or atheists embraced that term which was an epithet); rather they considered themselves true Christians or "rational Christians." Yet, given their system denied original sin, the trinity, incarnation, atonement, eternal damnation, infallibility of the Bible and saw non-biblical religions as valid ways to God, arguably it was not "Christianity." At least Adams' belief system was not "Christianity" as defined by its historic standards of orthodoxy.