When we say that God is a spirit, we know what we mean, as well as we do when we say that the pyramids of Egypt are matter. Let us be content, therefore, to believe him to be a spirit, that is, an essence that we know nothing of, in which originally and necessarily reside all energy, all power, all capacity, all activity, all wisdom, all goodness.

Often misquoted as "God is an essence that we know nothing of" and attached to part of another Adams letter to Jefferson (22 January 1825 — over five years later) on a different theological point, to form "God is an essence we know nothing of. Until this awful blasphemy is gotten rid of there will never be any liberal science in the world." Misquotation cited in and in The Quotable John Adams (Globe Pequot, 2008), p. 185ff.