From Acts of the Apostles, by William S. Kurz, SJ, commenting on Acts 9:26-31
After his arrival in Jerusalem, Saul tried to join the disciples. The Jerusalem disciples naturally avoided him out of fear, not believing that he was a disciple.
It took the mediation of Barnabas to incorporate Saul into the Jerusalem church. Barnabas took charge of Saul to facilitate his acceptance and introduced him to the apostles, who were still leaders in Jerusalem. He related to them the story of Saul’s vision of the Lord and his subsequent bold witness in the name of Jesus. Paul considered it essential to remain in unity with the original apostles (see especially Acts 15 and Paul’s collection from Gentile Christians for the poor of the Jerusalem community in 1 Cor 16:1–3).
Because of Barnabas, Saul was thereafter able to associate freely with them in Jerusalem. There he spoke out boldly in the name of the Lord, as he had done in Damascus.
The focus of this account, which highlights Paul’s acceptance by the apostles in Jerusalem, differs from Paul’s emphasis in Gal 1:15–20, where to some degree he minimizes his contact with the apostles in Jerusalem in order to emphasize that his apostleship came directly from Christ and was therefore as authentic as that of the Twelve. Although details of the accounts here and in Galatians 1 are difficult to harmonize, the main facts of the two reports can in general be reconciled.
It is unclear to what extent Paul’s fifteen-day private visit with Peter and James (Gal 1:18) may coincide with Barnabas’s introduction of Saul to the apostles and Saul’s witness to Jesus in Jerusalem described here in Acts 9. In any case, Saul’s bold speech in Jerusalem was not to last long, as the following verses make clear.
When Saul spoke and debated with the Hellenists, they reacted the same way as those in Damascus: they tried to kill him. But also as in Damascus, this plot against Saul became known. Therefore his fellow Christians took him down to Caesarea, and in that port city they put him on a ship to Tarsus, his hometown in the province of Cilicia in Asia Minor.