In discussing George Washington’s first Thanksgiving Day proclamation, his biographer James Flexner refers to Washington’s first inaugural address. About the inaugural address, he says:
The most remarkable aspect of what Washington wrote is the depth of its religious tone. He had often in the past expressed gratitude for the assistance of Providence to the American cause and had expressed hope that the boon would be continued. But never before had he devoted so much — more than a third — of a complicated pronouncement to religious considerations. That he was not just striking a popular attitude as a politician might is revealed by the absence of the usual Christian terms: he did not mention Christ or even use the world “God.” Following phraseology of the philosophical Deism he professed, he referred to “the invisible hand which conducts the affairs of men,” to “the benign parent of the human race.”
Flexner adds in a footnote:
That Washington intentionally avoided the word “God” is strongly indicated by his first Thanksgiving Proclamation. Having quoted Congress’s request that he establish a day for thanking “Almighty God,” in the part of the proclamation he himself wrote he used other designations.