If I had to summarize Paul’s gospel, I’d use Luke’s last scene in Acts, and I would suggest this text summarizes the missional theology of Paul in the Book of Acts:

28:30 Paul lived there two whole years in his own rented quarters and welcomed all who came to him, 28:31 proclaiming the kingdom of God and teaching about the Lord Jesus Christ with complete boldness and without restriction.

Paul’s gospel is summed up in two expressions: kingdom of God and Lord Jesus Christ. The gospel is about the society where God’s people do God’s reigning will and embody his will in a redemptive way, particularly for Paul now in contrast to the Roman empire’s way of doing things. And, the gospel is all about Jesus, the one from Nazareth; that Jesus is the Messiah, which sums up the entire plan of God through Israel’s Story; and that Messiah Jesus is now the crucified and resurrected and ruling Lord, no matter what kind of statues and cult are being worshiped and observed in the Roman forum — Paul’s gospel, then, strikes at the core of what makes Rome tick, and it strikes into that with force and power through the power of the Spirit.