Though I don't often see eye-to-eye with Grove City College on many cultural issues, I agree with what Dr. Gary S. Smith of that college concludes on Washington's faith:
I conclude therefore that Washington’s faith is better explained by the label “theistic rationalism” than by deism, Unitarianism, or Christianity. This theoretical construct combines elements of natural religion, Christianity, and rationalism, with rationalism predominating. It holds that God is unitary and active in the world and asserts that revelation complements reason. Since he directs human affairs, prayer is effectual. Because deists deny God’s active involvement in the world, the value of prayer, and the Bible as God’s revelation, the concept of theistic rationalism better describes Washington’s views than does the term deist, Unitarian, or Christian.
Smith, no doubt has been influenced by Dr. Gregg Frazer.