The reason that the life and message of Jesus Christ portrayed in the gospel accounts, particularly the Synoptics, seem so far removed from the post-resurrection Gospel of Paul and the other apostles to our thinking is that we wrongly see a chasm between the kingdom of God and the Gospel of Jesus Christ. The former is actually the foundation and presupposition of the latter.
Jesus the Messiah embodies the in-breaking of God‘s kingdom evidenced most graphically by healings and exorcisms, which dominate the Synoptics right up the passion narratives.
Jesus incarnates God’s good news of vanquishing sin in the world and rescuing sinners. “Trust in the crucified and risen Lord” is possible because of the message “Repent, for the kingdom of God is at hand.”
Paul’s message summarized in 1 Corinthians 15:1f. is the focused intensification of the broad, sweeping truth that in Jesus Christ God is reversing what man lost in Eden and bringing the entire world back into line with God’s holy purposes.
Never feel perplexed that the gospels are almost filled with accounts of healings and exorcisms, while Acts and the rest of the New Testament seem to go in an entirely different direction, with the stress on salvation by grace through faith in Jesus Christ.
The latter is the result of the former, is the natural consequence of the former, and impossible without the former.
The gospel of Jesus Christ is the gospel of the kingdom of God. No kingdom, no Gospel.