In a section examining Martin Luther’s Lectures on Hebrews (April 1517), Thomas J. Davis discusses how Luther began to mature theologically by working “extensively through the concepts of personal faith and the word of testament (the promise) and how the two are related in such a way as to portend his mature beliefs.” To that point, Davis observes:

Faith is a clinging to the Word of God for Luther. It is the only work of the Gospel, and it is internal. In a telling passage, Luther declared, “Without faith it is impossible for God to be with us.” Why is this so? Because God “does everything through the word.” For that Word to bear fruit for the believer, there must be faith. One can ties this arrangement to the incarnation. If Christ is God incarnate, as Luther certainly affirmed, the way one possesses that incarnate Word is through faith. The union of believers with the incarnate Word, a union so real that Luther speaks of Christ as the Christian’s substance, is achieved only through faith. If the Sacrament is a visible word, as Augustine of hippo taught and Luther accepted, then the Word itself is an audible body, Christ’s substance, possessed through the hearing and believing of it–through faith” (Thomas J. Davis, This is My Body: The Presence of Christ in Reformation Thought, 33).