>>>I don't see any mention of Christ or Christianity in Washington's Thanksgiving Day Proclamation of 1789. Would you please point out what you believe to be an invocation of Christianity in the 1789 Proclamation?<<<
They were all Christians, and there was never any doubt to which God Washington was referring ("with slight shades of difference, you have the same Religeon, Manners, Habits & political Principles." -- G.W.'s Farewell Address).
The proclamation actually originated in the House of Representatives, and was presented to the president by a committee that included Roger Sherman. After the proclamation, the day was widely celebrated throughout the nation, in both churches and capitals.