Laudator Temporis Acti

A quotation from Gerhard Tersteegen (1697-1769) is often repeated:

Ein begriffener Gott ist kein Gott.

I.e. "An understood God is no God." I can't locate the original. Cf. Tersteegen's Unparteiischer Abriss christlicher Wahrheiten (Spelldorf: Bernh. Rosshof und Comp., 1801), p. 25 (chap. 2, quaest. 14: "Was lehret dich diese Eigenschaft Gottes?"):

Gott zu bewundern, anzubeten, und mich im Geist und in der Wahrheit zu verlieren in diesem göttlichen Ungrund: und mich darüber zu erfreuen, daß wir einen so großen, so herrlichen und vollkommenen Gott haben, daß er nicht kann von einem endlichen Verstande begriffen werden.

This means something like:

What does this property of God teach you?

To admire God, to worship him, and to lose myself in spirit and in truth in this divine abyss: and to be glad that we have such a great, magnificent, and perfect God, that he can't be understood by a finite intellect.

In quaestio 13 he asks "Warum wird Gott unbegreiflich gennant?" (Why is God called incomprehensible?). "This property of God" in quaestio 14 is therefore his incomprehensibility.

I suspect that most people who quote "Ein begriffener Gott ist kein Gott" do so at second hand, probably from its mention by Rudolf Otto in The Idea of the Holy. I came across the sentence in ‎Robert B. Palmer's introduction to Walter F. Otto, Dionysus: Myth and Cult (Bloomington: University of Indiana Press, 1965), p. xix:

Applied to religion, this principle could easily echo Tersteegen's statement: "Ein begriffener Gott ist kein Gott" (A god who is understood is no god).

Augustine said something similar in his Sermon 117:

si comprehendis, non est Deus.