Question: Just as there is only one way to salvation, the crucified Christ, there is also only one doctrine of salvation that freely and graciously offers us Christ and all His benefits for our possession. What is that doctrine?
Answer: The gospel. Because it contains promises of salvation, it is called the gospel of salvation, a word of salvation, and a power of God unto salvation (Romans 1:16). “For I am not ashamed of the gospel, for it is the power of God for salvation to everyone who believes, to the Jew first and also to the Greek.” Of course the law leads us by the hand, as it were, to this doctrine. For after we are convicted of our unrighteousness and smitten with the awareness of eternal death, the law teaches us not to seek salvation in ourselves but to accept by faith the salvation offered us outside of ourselves in the gospel. St. Paul speaks of this in Romans 10:4 The end of the law is Christ for righteousness to every believer.” Also, “The law was a tutor to bring us to Christ” (Galatians 3:24).
Caspar Olevianus, A Firm Foundation, p. 8