Washington was not a Christian believer.
“George Washington’s adopted daughter, having spent twenty years of her life in his presence, declared that one might as well question Washington’s patriotism as question his Christianity. Certainly, no one questions his patriotism; so is it not rather ridiculous to question his Christianity? George Washington was a devout Episcopalian; and although as an Episcopalian he would not be classified as an outspoken and extrovert “evangelical” Founder as were Founding Fathers like Benjamin Rush, Roger Sherman, and Thomas McKean, nevertheless, being an Episcopalian makes George Washington no less of a Christian.”
http://christiananswers.net/q-wall/wal-g011.html
That does not make him an athiest.