John Calvin, Institutes of the Christian Religion (1.13.22):
…let us use great caution that neither our thoughts nor our speech go beyond the limits to which the Word of God itself extends. For how can the human mind measure off the measureless essence of God according to its own little measure, a mind as yet unable to establish for certain the nature of the sun’s body, though men’s eyes daily gaze upon it? Indeed, how can the mind by its own leading come to search out God’s essence when it cannot even get to its own? Let us then willingly leave to God the knowledge of himself. For as Hilary says, he is the one fit witness to himself, and is not known except through himself.…
And let us not take it into our heads either to seek out God anywhere else than in his Sacred Word, or to think anything about him that is not prompted by his Word, or to speak anything that is not taken from that Word.
Words of Grace 