Are you sure? He definitely said and wrote things to the contrary, including the Declaration of Independence.
I never doubted, for instance, the existence of the Deity; that He made the world, and governed it by His providence; that the most acceptable service of God was the doing good to man; that our souls are immortal; and that all crime will be punished, and virtue rewarded, either here or hereafter.
I have no dog in this race, being neither American nor religious, but it seems like an important historical detail.
I think we can agree he was a critic of organized religion, and that it's pretty enlightened to not have the state impose religion on anyone.
That said I still think he (at least for a large chunk of his life) believed in the existence of a god, the god of Abraham/Christianity in particular.