In the book of Acts, Luke introduces Saul dramatically as the someone who not only participated in the stoning of Stephen, but as an authority who gave approval for the execution. After Stephen delivers a prophetic speech in Acts 7 and describes his fellow Jews as a stiff-necked generation, like the tribes of Israel in the wilderness. He is seized by an angry crowd, taken outside the city and stoned. This is not a legal action, it is a lynching! Saul approved of this execution (Acts 8:1). Whether Saul was a legal representative of the Sanhedrin is unclear, but the verb can be used for legal approval (see, for example, 1 Maccabees 1:57). Luke describes Saul as “ravaging the church” (λυμαίνω, Acts 8:3), a word which is used of violent actions in war (Josephus, JW 4.534). Much later Paul described his persecution of the church as being zealous for the traditions of his Fathers (Gal 1:14). Like Elijah against the priests of baal, Paul burned with zeal and tried to destroy the church (Gal 1:22-23, Phil 3:6).
One problem for modern readers is a misunderstanding about what Saul was doing. We tend to read modern persecution of Christians into the passage, or maybe lurid scenes from old movies of Nero throwing the saints to the lions. Nor should we think of rabbi Saul like a Puritan going going door-to-door to root out the heretics. Stephen was teaching Jesus was the messiah and in some way replaces worship in the Temple. The High Priest executed Jesus unjustly but God raised Jesus from the dead, proving he was in fact the messiah. Saul saw all this as a dangerous defection from the Law and could result in the judgment of God.
What was it about Stephen’s speech that pushed Saul to such a violent response?
Some scholars explain this violent reaction by taking later issues and forcing them into Acts 7. For example, some have argued the Jewish Christians were accepting Gentiles into the synagogue without circumcision. This seems unlikely since there is no reference at all to Gentile mission by the Jerusalem Church until Acts 10. God-fearering Gentiles were accepted into the synagogue without circumcision, so it is unlikely this would be a problem for Paul.
Similarly, some argue Gentile believers were breaking food laws. This is unlikely for the same reasons as the first. There is no evidence of Gentile converts in the pre-Pauline period. Jewish and Gentiles sharing meals is a problem in the book of Galatians, but that is as many as fifteen years after the stoning of Stephen.
A more likely motivation is the possible political and social problems caused by the preaching of a crucified messiah. How would this sound to the Gentiles, especially the Romans? Could this be an accusation against and a possible rally-point for anti-Roman activity? The problem here once again is the lack of evidence for preaching anything to Gentile audiences. The early apostolic mission prior to the stoning of Stephen was confined to the temple area and the city of Jerusalem in general.
It is best to see Saul opposing the Apostolic teaching as a false and potentially dangerous teaching about the messiah. For Rabbi Saul, the very idea Jesus was the Messiah was absurd. Because Jesus was crucified (“hung on a tree”) he was under the curse of the Law. Saul’s motivation is to correct this false teaching within Judaism, using the synagogue punishment system itself. Paul sees himself as a reformer within Second Temple Judaism. He represents the high priest (who approved of the execution of Jesus) and reacted with violence against those who argued this the condemned rabbi Jesus was the Messiah and that worship of Jesus is superior to worship in the Temple.