When we say 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.

Letter to Thomas Jefferson (17 January 1820). Often misquoted as "God is an essence that we know nothing of" and attached to a part of his 22 January 1825 letter to Thomas Jefferson.