All were pro-Christianity because they were "pro-religion" in general. They held the purpose of "religion" or "Christianity" was to make men moral (not necessarily to be saved through Christ's blood atonement). Works were more important than faith for salvation. The test of "sound" religion was that it in fact made men moral. As such if the "ends" (morality) were achieved, the means (which religion you are) really didn't matter. But Christianity had an edge over other world religions, not because of Jesus' exclusive claim to God or His status as second person in the Trinity, but because He was the greatest moral teacher. Yet, they still saw a place at the table of "sound religion" for virtually all of the world's religions as valid ways to God. Their creed, I believe, was key in making America a haven for non-Christian religions.
Further, theirs was a form of hyper-Arminianism that was theologically unitarian and universalist. In short, the "Protestantism" of Washington et al. was precisely the kind of "Christianity" that orthodox theologian John MacArthur rails against here. This is why if orthodox Trinitarian Christians truly understood what Washington and the other key Founders believed they'd either call it "heresy" or, like Mormonism, "not Christianity." That's why orthodox Christian scholar of America's Founding, Dr. Gregg Frazer, terms their belief system "theistic rationalism," not "Christianity."
First quotations from other key Founders who are not George Washington. Then, Washington's:
"My fundamental principle would be the reverse of Calvin's, that we are to be saved by our good works which are within our power, and not by our faith which is not within our power."
-- Thomas Jefferson to Thomas B. Parker, May 15, 1819.
"No point of Faith is so plain, as that Morality is our Duty; for all Sides agree in that. A virtuous Heretick shall be saved before a wicked Christian."
-- Benjamin Franklin, "Dialogue between Two Presbyterians," April 10, 1735.
"...the design of Christianity was not to make men good Riddle Solvers or good mystery mongers, but good men, good magestrates and good Subjects...."
-- John Adams, Dairy, Feb. 18, 1756
"Faith is recommended as a Means of producing Morality: Our Saviour was a Teacher of Morality or Virtue, and they that were deficient and desired to be taught, ought first to believe in him as an able and faithful Teacher. Thus Faith would be a Means of producing Morality, and Morality of Salvation. But that from such Faith alone Salvation may be expected, appears to me to be neither a Christian Doctrine nor a reasonable one....Morality or Virtue is the End, Faith only a Means to obtain that End: And if the End be obtained, it is no matter by what Means."
-- Benjamin Franklin, "Dialogue between Two Presbyterians," April 10, 1735.
"Here is my Creed: I believe in one God, Creator of the Universe. That He governs it by his Providence. That he ought to be worshipped. That the most acceptable Service we can render to him, is doing Good to his other Children. That the Soul of Man is immortal, and will be treated with Justice in another Life respecting its Conduct in this. These I take to be the fundamental Principles of all sound Religion, and I regard them as you do, in whatever Sect I meet with them."
-- Benjamin Franklin to Ezra Stiles, March 9. 1790.
"Every religion consists of moral precepts, and of dogmas. In the first they all agree. All forbid us to murder, steal, plunder, bear false witness &ca. and these are the articles necessary for the preservation of order, justice, and happiness in society. In their particular dogmas all differ; no two professing the same. These respect vestments, ceremonies, physical opinions, and metaphysical speculations, totally unconnected with morality, and unimportant to the legitimate objects of society. Yet these are the questions on which have hung the bitter schisms of Nazarenes, Socinians, Arians, Athanasians in former times, and now of Trinitarians, Unitarians, Catholics, Lutherans, Calvinists, Methodists, Baptists, Quakers &c. Among the Mahometans we are told that thousands fell victims to the dispute whether the first or second toe of Mahomet was longest; and what blood, how many human lives have the words 'this do in remembrance of me' cost the Christian world!...We see good men in all religions, and as many in one as another. It is then a matter of principle with me to avoid disturbing the tranquility of others by the expression of any opinion on the [unimportant points] innocent questions on which we schismatize, and think it enough to hold fast to those moral precepts which are of the essence of Christianity, and of all other religions."
-- Thomas Jefferson to James Fishback, Sept. 27, 1809
"I believe with Justin Martyr, that all good men are Christians, and I believe there have been, and are, good men in all nations, sincere and conscientious."
-- John Adams to Samuel Miller, July 8, 1820.
Now go and read every single time Washington praises the Christian religion and see that it is always in the context of equating Christianity with mere morality not orthodox Christian doctrine. The pietists use the following as "proof quotes" to show that Washington wasn't a Deist, therefore he was a "Christian." But what they actually show is that Washington was a "Christian" in the same way that J. Adams, Jefferson and Franklin were "Christians." My emphasis or italics will be on those parts of Washington's words that illustrate my contention.
“To the distinguished Character of Patriot, it should be our highest Glory to add the more distinguished Character of Christian. The signal Instances of providential Goodness which we have experienced and which have now almost crowned our labours with complete Success, demand from us in a peculiar manner the warmest returns of Gratitude and Piety to the Supreme Author of all Good [my emphasis]."
-- George Washington, General Orders, Saturday, May 2, 1778.
"While all men within our territories are protected in worshipping the Deity according to the dictates of their consciences; it is rationally to be expected from them in return, that they will be emulous of evincing the sanctity of their professions by the innocence of their lives and the beneficence of their actions; for no man, who is profligate in his morals, or a bad member of the civil community, can possibly be a true Christian, or a credit to his own religious society [my emphasis].
"I desire you to accept my acknowledgments for your laudable endeavours to render men sober, honest, and good Citizens, and the obedient subjects of a lawful government [my emphasis]."
-- George Washington, Letter to General Assembly of Presbyterian Churches, May 1789.
"Without reverberating the arguments in support of the humane and benevolent intention of Lady Huntington to christianize and reduce to a state of civilization the Savage tribes within the limits of the American States, or discanting upon the advantages which the Union may derive from the Emigration which is blended with, and becomes part of the plan, I highly approve of them,...[my emphasis]."
-- George Washington To SIR JAMES JAY, January 25, 1785.
"In the meantime, it will be a desirable thing for the protection of the Union to co-operate, as far as circumstances may conveniently admit, with the disinterested endeavours of your Society to civilize and Christianize the Savages of the Wilderness [my emphasis]."
-- George Washington to The Society of the United Brethren for Propagating the Gospel among the Heathen, July, 1789.
Finally, keep in mind that Washington never spoke in orthodox Trinitarian language, but used generic philosophical titles for God. Never said "Father, Son, Holy Spirit," never used the word "Redeemer" (these were Congress' not Washington's words!), systematically avoided discussing Jesus by name or example as though he didn't care about or had no personal relationship with Jesus, and systematically avoided communion in his Church to which he nominally belonged.