Paul never left Judaism, Stendahl argued, he remained a faithful Jew who was fulfilling the role of being the “light to the Gentiles” from Isaiah. In this view, Paul received a new calling, but still served the same God. He was to remain a Jew who was called by God to be the witness to the gentiles as anticipated in the prophecies of Isaiah. Paul is therefore not “founding a new religion” but rather a new understanding of the Jewish Law. His gospel is a new interpretation of the Hebrew Bible and Judaism, he simply changed parties within Judaism.
The problem with this new view of Paul’s experience on the road to Damascus is that it does not do justice to the radicalness of Paul’s Gospel! To reject circumcision even for Gentile converts is not a minor re-interpretation of the Jewish Law, it is a radical change that is unanticipated in the prophets.
The reaction of the Jews in Acts is key. Everywhere Paul announces that God has called the Gentiles to be saved without circumcision, they riot and attempt to kill Paul. Philippians 3:7-8 make it clear that Paul is not just moving to another party within Judaism, but rather that he is rejecting his Pharisaic roots completely. He is breaking with his past way of life and his past theology. While there are many points of comparison between Paul’s theology and Judaism, there are far more radical breaks with the Judaism of the first century.
However, I do think that it is problematic to think that Paul is converting from Judaism to Christianity. Paul seems rather clear in Galatians that he was called by God to be the apostle to the Gentiles in a way that is quite distinct from the apostles in Jerusalem that were called by Jesus. He stresses his independence clearly in Galatians. He never joins the Jerusalem church, nor does he receive his commission from them, but he seems to be called by God to do something quite different – to be the apostle to the Gentiles. Despite the expansion of the apostolic witness to Hellenistic Jews and God-Fearers, the Twelve do not appear in Acts to do ministry outside of the house of Israel. Galatians 1-2 seems to be saying that there was a tacit agreement between Paul and Peter marking the “boundaries” of their ministerial territory. Paul will go to the Gentiles and Peter to the Jews.
It is probably best to see Paul’s Damascus Road experience as both a conversion and a call. But to think of the categories “conversion” and “call” in modern Christian categories is a mistake, Paul’s experience in Acts 9 is quite unique in salvation history.