Dr. Martin Luther, as quoted in One True God: Understanding Large Catechism II.66:
Reason plays the game of blind man’s bluff with God. It makes nothing but false moves and always misses Him in that it calls that “God” which is not God; and, conversely, fails to call Him God who is God…. It goes about the matter in this clumsy way, ascribing the name of God and divine honor to what it imagines to be God but never hitting upon the true God but rather on the devil or its own notion, which the devil controls. This is the reason why there is indeed a very great difference between knowing that God exists and knowing what or who God really is. The first truth human nature knows, and it is written in all hearts; the second only the Holy Spirit teaches. (St. L 14:859)
Edward Engelbrecht, One True God: Understanding Large Catechism II.66 (St. Louis: CPH, 2007) 98.