You may ask, “What then is the Word of God, and how shall it be used, since there are so many words of God?” I answer: The Apostle explains this in Romans 1. The Word is the gospel of God concerning his Son, who was made flesh, suffered, rose from the dead, and was glorified through the Spirit who sanctifies.
To preach Christ means to feed the soul, make it righteous, set it free and save it, provided it believes the preaching. Faith alone is the saving and efficacious use of the Word of God alone, according to Romans 10:9: “If you confess with your lips that Jesus is Lord and believe in your heart that God raised Him from the dead, you will be saved.” Furthermore, Christ is the end of the law, that everyone one who has faith may be justified” [Romans 10:4]. Again, in Romans 1:17, “He who through faith is righteous shall live.” The Word of God cannot be received and cherished by any works whatever but only by faith. Therefore it is clear that, as the soul needs only the Word of God for its life and righteousness, so it is justified by faith alone and not any works; for if it could be justified by anything else, it would not need the Word, and consequently it would not need faith.
– Martin Luther, On Christian Liberty (Trans. W.A. Lambert; Minneapolis, MI: Fortress Press, 2003), 7-8.