The Gospel in All Its Forms
Simon Gathercole distills a three-point outline that both Paul and the Synoptic writers held in common. (See "The Gospel of Paul and the Gospel of the Kingdom" in God's Power to Save.) He writes that Paul's good news was, first, that Jesus was the promised Messianic King and Son of God come to earth as a servant, in human form. (Rom. 1:3-4; Phil. 2:4ff.)
Second, by his death and resurrection, Jesus atoned for our sin and secured our justification by grace, not by our works (1 Cor. 15:3ff.) Third, on the cross Jesus broke the dominion of sin and evil over us (Col. 2:13-15) and at his return he will complete what he began by the renewal of the entire material creation and the resurrection of our bodies (Rom 8:18ff.)
Gathercole then traces these same three aspects in the Synoptics' teaching that Jesus, the Messiah, is the divine Son of God (Mark 1:1) who died as a substitutionary ransom for the many (Mark 10:45), who has conquered the demonic present age with its sin and evil (Mark 1:14-2:10) and will return to regenerate the material world (Matt. 19:28.)
If I had to put this outline in a single statement, I might do it like this: Through the person and work of Jesus Christ, God fully accomplishes salvation for us, rescuing us from judgment for sin into fellowship with him, and then restores the creation in which we can enjoy our new life together with him forever.
One of these elements was at the heart of the older gospel messages, namely, salvation is by grace not works. It was the last element that was usually missing, namely that grace restores nature, as the Dutch theologian Herman Bavinck put it. When the third, "eschatological" element is left out, Christians get the impression that nothing much about this world matters. Theoretically, grasping the full outline should make Christians interested in both evangelistic conversions as well as service to our neighbor and working for peace and justice in the world.