By about 1515 (Luther) had evolved the essential outlines of his theology. He termed it "a theology of the cross." For only at the cross of Christ do we experience God in a manner that stands the test of reality. God must send us into darkness and into trials and temptations to free us of ourselves and to prepare us for the reception of His grace, which faith, contrary to all expectations, finds solely at the cross.
"Luther's World of Thought"
Heinrich Bornkamm, pages 3-4