Who he was
A Jewish Pharisee (Phil 3:5-6; Gal 1:14; 2 Cor 11:22) who believed in the strict observance of God's Law both for himself and for all believing Jews.
Why he became a Christian
Journeying to Damascus Paul had an encounter with the risen Christ (Acts 9: 1-19). He became convinced that fellowship with the risen Jesus (Gal 1:11-12), not the observance of the Law (Gal 3:1-5), was the necessary and sufficient condition for receiving and participating in God's promise for salvation.
Paul believed that Christ had come for all and the observances of Jewish Law were not necessary for those who believed in Christ (Gal 2:15-21 - 3:1-5). This created an ongoing battle with those who thought that Jewish observances were necessary for all those who sought entrance into the Church (Gal 5:7-12).
St. Paul's Essential Message
In Jesus Christ God had acted to provide salvation for all who believe (Rom 1: 1-7). This salvation, whose complete realization lay in the future, has its beginnings in the present. People can experience this salvation in their own lives (Rom 8:14-17).
Centrality of Jesus Christ
Christians are united with Christ through faith (Rom 5: 1-2) and rejoice in the gift of God's Spirit, waiting for the time when the Lord will return and the work of salvation complete (1 Thess 4:13-18).
The Christian's Bond with Christ
Through faith and baptism (Rom 6: 3-5) the Christian assumes a new identity in Christ. On the social level, the Christian joins the community of the church, which proclaims the Gospel and lives in union with Christ's Spirit (Gal 3: 25-28). On the personal level, Christians recognize that, united with the Lord and justified by Christ, they are given the help needed to overcome any tendency for immoral living (Galatians 5:16-26).
In Paul's thought the justice of God was salvific justice at its best. God is faithful, fulfilling the promises made in the covenant. God justifies, which means that it is God alone who reconciles humankind through Christ (Rom 3:21-31). People cannot justify themselves, but can only be justified by being united in faith with Jesus Christ and by accepting the grace won by Christ (Rom 5:1-2). Justification is not something that can be won by following the Law, but people can only be made right with God and set free from a life of immorality by accepting God's reconciling grace as a free gift.
The Wisdom of God
The notion that God's saving work is accomplished in the crucified Jesus is a scandal to the Jews and foolishness to the Greeks (1 Cor 1: 17-25). The idea that a condemned person could be a vehicle of salvation went against basic tenets of the Jewish faith. They could not believe that a crucified man could be a sign from God. It is foolishness to the Greeks because the idea that the transcendent God would be involved in human affairs was ludicrous, especially in the form of a crucified criminal.