As you can see the dispute over the Founding Fathers' religion has been going on for some time. Even though the letter was written to the New York Times in 1897, the points are still apt. Moncure Conway was a freethinker who did some notable scholarship on George Washington's religion, in particular his lack of Christian orthodoxy.
Even though Conway terms Washington a "Deist," the evidence he then cites, while it does point away from Washington's orthodox Christianity, also somewhat belies the notion that GW was a strict Deist. For instance, Deists don't tend to think any of the Bible is a "benign light," yet unitarians and "Christian rationalists" do believe parts of scripture are benign and enlighted, and it's only those parts in which they tended to believe. Conway also terms Washington a Socinian. Socinians are not Deists, but Unitarians who believe Jesus was not God but 100% man on some kind of divinely inspired moral mission. The 1783 Circular to the States to which Conway refers was not written in Washington's hand but was signed by him. It refers to Jesus as "the Divine Author of our blessed Religion," and if unitarian, seems more Arian, which believes Jesus a divine but created and subordinate being, than Socinian, which views Jesus as only human. This and in one other public address to Delaware Indians, neither written in Washington's hand, but both signed by him, are the only two places Washington discusses the name or person of Jesus at all! This makes it a little tough for those of us who want to with certainty place Washington in a religious box. James Madison and a number other of Washington's contemporaries noted, other than believing in an active personal God Washington seemed not to have formed definite opinions on Christian or other theologies. So it's entirely possible that whereas Jefferson and others actively disbelieved in doctrines like the Trinity, Washington was agnostic on those matters. In any event, let me reproduce the passage where GW appeals to revelation for authority, one of the few places he does so. It is done in an enlightenment rationalistic context:
The foundation of our Empire was not laid in the gloomy age of Ignorance and Superstition, but at an Epocha when the rights of mankind were better understood and more clearly defined, than at any former period, the researches of the human mind, after social happiness, have been carried to a great extent, the Treasures of knowledge, acquired by the labours of Philosophers, Sages and Legislatures, through a long succession of years, are laid open for our use, and their collected wisdom may be happily applied in the Establishment of our forms of Government; the free cultivation of Letters, the unbounded extension of Commerce, the progressive refinement of Manners, the growing liberality of sentiment, and above all, the pure and benign light of Revelation, have had a meliorating influence on mankind and increased the blessings of Society. At this auspicious period, the United States came into existence as a Nation, and if their Citizens should not be completely free and happy, the fault will be intirely their own.