N in an attempt to defend his former ministry among the uncircumcised gentiles, Paul spoke in Hebrew to his Jewish persecutors. Simply put, Paul gave his testimony of salvation & of his call into the gospel ministry. However, he did specifically emphasize his Jewish heritage, speaking plainly about what he had in common with his Jewish persecutors. He shared with them how Gamaliel had trained him in all matters of the law. He gave credit to the Jews for their religious zeal (vs. 3). He admitted that (prior to his salvation) he had been exactly where they were. He understood their perspective. But something had happened which had changed his life. Obviously, he wanted them to go through that same transformation; not just salvation, but also an acceptance of the broadness of God's grace to the gentiles, without the law.
It is significant that the crowd did not erupt when Paul spoke of his vision of Jesus of Nazareth (from when he had been on the road to Damascus). Note that at least a part of this crowd was comprised of Messianic Jews, having believed that Jesus was indeed the Messiah, the Son of God, the Savior of the world. Remember, when Paul had arrived in Jerusalem he had been warned by the elders of the church that a large portion of the church there was still zealous concerning the Old Testament law, and that they were violently intolerant of Paul's doctrine concerning liberty from the law (Acts 21:20-22). As such, it wasn't Paul's faith in Christ that bothered them. Paul continued with his testimony about being healed by Ananias (another law keeping believer), and about being baptized and forgiven in the name of Christ the Lord (Acts 22:16). The crowd remained quiet and attentive throughout this narrative. But, when Paul got into his calling; when he told the crowd that he had been called of Christ (while praying in the temple, no less) to minister (as he had) among the uncircumcised heathen, the Jews' patience evaporated. Immediately, they demanded his death (Acts 22:21-22). At least some of these people were followers of Christ; or claimed to be (see the entire book of Hebrews)! Isn't it amazing that they could be so full of vitriol and hatred for this good apostle!? Yet, they were.