Smithsonian: "Why No One Can Agree on What George Washington Thought About the Relationship Between Church and State"

By a professor at Stanford. A taste:

Historians were not deaf to Washington’s religious references. While the clergy and the scientists saw them as evidence of Washington’s devotion, the historians stressed the president’s precision in crafting a vocabulary that would unite the dizzying array of Protestant denominations in post-revolutionary America without alienating the small but important groups of Catholics, Jews, and freethinkers dotting the American landscape. It was precisely because he understood that Americans did not believe the same thing that Washington was scrupulous in choosing words that would be acceptable to a wide spectrum of religious groups.

In his own time, Washington’s reluctance to show his doctrinal cards dismayed his Christian co-religionists. Members of the first Presbytery of the Eastward (comprised of Presbyterian churches in Massachusetts and New Hampshire) complained to the president that the Constitution failed to mention the cardinal tenets of Christian faith: “We should not have been alone in rejoicing to have seen some explicit acknowledgement of the only true God and Jesus Christ,” they wrote. Washington dodged the criticism by assuring the Presbyterians that the “path of true piety is so plain as to require but little political direction.”  

Similarly, a week before his 1789 proclamation, Washington responded to a letter from Reverend Samuel Langdon, the president of Harvard College from 1774-1780. Langdon had implored Washington to “let all men know that you are not ashamed to be a disciple of the Lord Jesus Christ.” Once again, instead of affirming Christian tenets, Washington wrote back offering thanks to the generic “Author of the Universe.”