Thomas Jefferson was frequently accused of being an atheist (I tend towards those who suppose him to be a particularly secular Deist), but usually avoided commitment and included references to God (or someone similar) in his writings, particular the more or less public (I have been reading so much 18th century English writing that I almost spelled that ‘publick’) ones. An obvious example is the capital C Creator referenced in The Declaration of Independence.
But in a letter to his nephew, Peter Carr, regarding a proper course of study, he encourages a critical examination of Scripture and acknowledges that such an examination could lead him to atheism (though he doesn’t use that word). Then he lays out as well as any(and succinctly than most) contemporary atheist thinkers why atheism (at least, educated atheism) will naturally lead to moral behavior.
Do not be frightened from this inquiry by any fear of its consequences. If it ends in a belief that there is no God, you will find incitements to virtue in the comfort and pleasantness you feel in its exercise, and the love of others which it will procure you. If you find reason to believe there is a God, a consciousness that you are acting under his eye, and that he approves you, will be a vast additional incitement; if that there be a future state, the hope of a happy existence in that increases appetite to deserve it; if that Jesus was also a God, you will be comforted by a belief of his aid and love.