It is not so much that God cannot be seen that concerns Luther, but that God actually and actively hides. God hides in order not to be found where humans want to find God. But God also hides in order to be found where God wills to be found.
Word & World
Paulson, Steven D. “Luther on the Hidden God.” Word & World 19, no. 4 (September 1999): 363–71.
Paulson, Steven D., "Luther on the Hidden God" (1999). Faculty Publications. 160.