Heiko Oberman on Luther’s view of knowledge of God:
. . . Since the Fall every man has been a philosopher, for he has taken his experience of the world and his knowledge of reality — which he has succeeded in describing scientifically — as a standard by which to measure God. But the intellect does not suffice to grasp the God of Abraham, Isaac, and Jacob; He must be apprehended through the Scriptures. The “God” created by man is a false god of his own making. (Heiko Oberman, “Luther: Man Between God and the Devil,” 170)
Luther is not arguing against thinking or being “rational,” instead his context was disputing with the scholastics of his day; these were those who engaged the philosophy of his day, articulated by his compatriots in the Roman Catholic church. But his point of dispute was not so limited to these philosophers; instead he believed (as Oberman summarizes) that endemic to the Fall, man is bent in upon himself — and will create God in his own image. In this sense then, trying to reach or think about God, except from Christ’s revelation (Jn. 1:18), flows from man reflecting — without revelation — upon what is intuitive or apparent to him through his experience and his reflections upon this experience. It is in this framing that Luther would say that all men are philosophers.