Trying to summarize Paul under three headings: Paul the Jew, Paul the Christ-Follower, and Paul the Apostle. This is the first of "Paul the Christ-Follower."
Paul the Christ-Follower
Paul speaks of a time when God was pleased "to reveal his Son in me" (Gal. 1:16). In Acts 9 we get a fuller version of this event. On his way to Damascus, the risen Christ appeared to Paul and changed the trajectory of his life from someone zealous for the ethnic distinctives of the Jewish Law to someone who zealously proclaimed that Christ is king.
Like many Jews of his day, Paul may have already been looking for a messiah, a king, to come eventually to rule over a renewed nation of Israel. As a Pharisee, Paul would already have believed in a future resurrection at least for those killed because of their faithfulness to God. But initially, the idea that Jesus might be this king did not compute for Paul. Messiahs were not supposed to die, and the resurrection was supposed to happen all at once, not just to one individual.
Not only that, but this Jesus was not zealous for the Law the way he thought the messiah surely would be. Jesus ate with tax collectors and sinners. He did not seem to pay much attention to the rules about the sabbath or matters of purity. He included people who were unclean in his fellowship.
All of that changed when Christ appeared to him, and Paul had to rethink his entire way of understanding. Jesus was the Messiah, the "anointed one," the "Christ" in Greek. He now confessed that "Jesus is Lord," perhaps the earliest Christian confession of faith. To say Jesus was Lord was to say that God raised him from the dead and enthroned him as king of the universe (Ps. 110:1). This is the heart of the gospel, the "good news," for Paul: Jesus has arrived as God's king to set up God's kingdom. He is the one Lord who goes with the one God (1 Cor. 8:6). He will come again to earth soon to finish what he started, to judge the earth while rescuing those who confess him as Lord.
However, Paul tells us that the focus of his preaching was the cross (e.g., 1 Cor. 1:23). In the cross, like the believers before him, Paul saw Christ's death as a sacrifice for sins. Through the "faith of Jesus," God showed his people his continuing faithfulness to them. But unlike most believers before him, Paul argued that this sacrifice could apply both to Jews and Gentiles. He believed that God graciously would accept Gentiles because of Christ's death, as well as Jews.
In fact, Paul came to believe that God accepted Jews on exactly the same basis, that no amount of keeping of the Jewish Law could establish a right relationship between a Jew and God. After all, he had been blameless at keeping the Law and still needed Christ. God must obviously be looking for something other than Law keeping, and it turned out to be the cross of Christ.
Paul follows through with the new revelation. The Jewish Law put the Jews under a curse. It only showed their inability to keep it. The death of Christ redeemed them from that curse. God in his grace had decided to reconcile the entire world to himself through Christ, apart from things like circumcision, the purity and food laws, or sabbath observance. These are things Paul primarily had in mind when he said a person could not be made right with God or "justified" by "works of Law" (e.g., 3:28), the things that most separated Jew from Gentile. God was both the God of the Jews and the Gentiles (Rom. 3:29). He was the God of all who had faith in him (4:16), those who have trusted in what he has done through Christ (9:33). All who call on the name of the Lord Jesus will be saved from God's coming judgment (10:13).
Key for Paul was that a person participate and "get inside" of Christ's death and resurrection. We are baptized into Christ. We are buried with him in baptism (Rom. 6:4), and we rise with him to live a new life in his resurrection (6:8; 8:11). The life that we now live, we live "in the faithfulness of the Son of God" (Gal. 2:20, my translation). God accomplishes this transformation through the Holy Spirit, whom God has given us as both a guarantee and down payment of our inheritance that is to come (2 Cor. 5:5).
The result is that we now live a life "according to the Spirit" rather than "according to the flesh," which is how we used to live (Gal. 5:16; Rom. 8:8), a life of love toward our neighbor. Paul's ethic for his Gentile converts is basically the Jewish Law stripped of its ethnic particulars. External matters like circumcision and purity rules, even sabbath observance are not part of God's expectation for Gentile believers. However, they remain under Christ's law (e.g., 1 Cor. 9:21). They remain, for example, under the sexual prohibition of the Jewish Scriptures, which Christians later came to call the "Old" Testament.