A God comprehended is no God.

[Ein begriffener Gott is kein Gott.]

Gerhard Tersteegen (1697-1769) German religious writer, preacher, mystic, hymnist [Dutch, Gerrit ter Steegen]
(Attributed)

The earliest reference I can find is in an epigraph in Rudolph Otto, The Idea of the Holy [Das Heilige] (1917) [tr. Harvey (1924)]. This is where most citations point to.