What's the gospel?
4. God has fulfilled his promise to Israel's fathers in Jesus Christ. Paul told the Jewish synagogue at Pisidian Antioch that "we proclaim to you the good news about the promise to our ancestors, that this promise God has fulfilled to us, their children, by raising Jesus" (Acts 13:32-33).
The essence of God's promise that an Anointed One, a Messiah/Christ, would come. The gospel is that that Messiah has come, and it is Jesus (Acts 5:42). The gospel also includes what this Messiah did, not only rising from the dead, as Paul told the Pisidians, but also dying on the cross. At first hearing, the idea of a crucified Messiah didn't sound like good news to a Jew but something incomprehensible, a "stumbling block" (1 Cor. 1:23). But the apostles proclaimed the gospel to their fellow Jews by repeatedly demonstrating that "[t]his gospel [was] promised beforehand through [God's] prophets in the holy scriptures" (Rom. 1:2). Even though Jesus didn't fit the Jews' mental model of what a Messiah was supposed to be, that's because the gospel is a "mystery that had been kept secret for long ages" that just now has been revealed by God in Jesus' person and work (Rom. 16:25).
But the fact that God has made good on his promise to Israel isn't just good news for Jews but good news for Gentiles too—another shock to Jewish assumptions. "[T]his secret was not disclosed to people in former generations as it has now been revealed to [God's] holy apostles and prophets by the Spirit," Paul writes to the Ephesians, "namely, that through the gospel the Gentiles are fellow heirs [with Jews], fellow members of the body, and fellow partakers of the promise in Christ Jesus" (Eph. 3:5-6). When Gentiles respond to the gospel, they turn from the worthless idols they are accustomed to worship to "the living God, who made the heaven, the earth, the sea, and everything that is in them" (Acts 14:15).
For this reason, the gospel is both what Jesus the Promised Messiah did in his earthly life and what he continues to do through his entire Church, Jew and Gentile together. This is why Mark entitles his account of the life of Jesus "The beginning of the gospel of Jesus Christ, the Son of God" (Mark 1:1).
God has fulfilled his promise to Israel's fathers in Jesus Christ. That is good news.