15 So, as much as is in me, I am ready to preach the gospel to you who are in Rome also. 16 For I am not ashamed of the gospel of Christ, for it is the power of God to salvation for everyone who believes, for the Jew first and also for the Greek. 17 For in it the righteousness of God is revealed from faith to faith; as it is written, “The just shall live by faith.” (Romans 1:15–17)
Why preach the gospel? Because it is God’s power to save the lost. Why does the gospel possess such power? Because it reveals “the righteousness of God”. That is, the gospel reveals the means by which God counts sinners righteous; it is “by faith” (Rom. 5:1). Plus, the gospel produces personal faith in Jesus (Rom. 10:17). This is the thesis statement of Romans. The gospel teaches us how God saves sinners. The gospel teaches us about the personal faith we must have in Jesus in order to be saved. That is why the gospel must be preached.