Morality is Instinctual

Some have made the love of God the foundation of morality ... If we did a good act merely from the love of God, and a belief that is pleasing to Him, whence arises the morality of the Atheist? ... In protestant countries, the defections from the platonic Christianity of the priests is to Deism, in catholic countries they are to Atheism. Diderot, D'Alembert, D'Holbach, Condorcet [philosophers & atheists], are known to have been among the most virtuous of men. Their virtue, then, must have had some other foundation than love of God.
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Nature hath implanted in our breasts a love of others, a sense of duty to them, a moral instinct. ... I sincerely, then, agree with you in the general existence of a moral instinct. I think it the brightest gem with which the human character is studded.

Letter to Thomas Law, June 13, 1814