The gospel is good news about the coming of a King. This is plain in Isaiah, where the prophet gives us important background for understanding “gospel”: “How beautiful upon the mountains are the feet of him who brings good news, who publishes peace, who brings good news of happiness, who publishes salvation, who says to Zion, ‘Your God reigns'” (Isa. 52:7). In Isaiah 61:1-2, which Jesus quotes in the synagogue at Capernaum, we hear a similar gospel:
“The Spirit of the LORD GOD is upon me, because the LORD has anointed me to bring good news to the poor; he has sent me to bind up the brokenhearted, to proclaim liberty to the captives, and the opening of the prison to those who are bound; to proclaim the year of the LORD’s favor, and the day of vengeance of our God.”
Isaiah goes on to mention other things. But here, too, the gospel is about the coming of the Anointed One, the Messiah, the King, and all the things the King will do: bind up the brokenhearted, set captives free (who but a king can do that?), proclaim both God’s favor and his vengeance.
At the beginning of their ministries, both John the Baptist and Jesus proclaimed as gospel, “Repent, for the kingdom of heaven is at hand” (Matt. 3:2; 4:17). Again, the gospel is the coming a great King. The gospel is not just about us. It is not limited to justification by faith. It is focused on God and his coming. It is almost political in its force. To the Romans, the “gospel,” or good news, was that a new emperor had come into power. They proclaimed kyrios Caesar, “Caesar is Lord.” The Christians proclaimed kyrios Iesous, “Jesus is Lord.” You can understand why the Roman rulers became nervous. Of course, they misunderstood to some degree what kind of King Jesus was. But they were not wrong to feel threatened. King Jesus claims sovereignty over them. (Think of Ps. 2, where God calls the rulers of the world to kiss the anointed Son.)
Never forget that Jesus is Lord and King of all, and he will not accept any lesser position. He demands that we do all things to his glory, everything in accord with his will. His gospel contains law, we may say. But service to this King is wonderful freedom. To trust this King is to trust a Priest who gives us full forgiveness from God and constant intercession. And to trust this King is to trust a Prophet whose word is completely true and trustworthy.
John Frame, Salvation Belongs to the Lord: An Introduction to Systematic Theology, 157-158