What does Acts 26:19 mean?

The Sanhedrin accused Paul of leading a cult, desecrating a religious structure, and starting riots (Acts 24:5–6). After a cursory investigation, Governor Felix determined Paul was innocent, but kept him under house arrest to placate the Jewish leaders (Acts 24:27). Two years later, when Festus replaced Felix, the Sanhedrin asked Festus to reopen the case. Festus, too, realized Paul was innocent, but while trying to appease the Sanhedrin without annulling Paul's rights as a Roman citizen, he waffled too long, and Paul appealed to Caesar (Acts 25:1–12). Now, Festus must send Paul's case to Rome, but he has no charges to send.

Festus has decided to ask King Agrippa II for help. The great-grandson of Herod the Great, Agrippa has much more experience with Jewish customs and religion. He's intrigued by the case, and Festus invites him, the king's sister/lover Bernice, and the leaders of Caesarea Maritima to hear Paul's side of the story in hopes they can find something Festus can tell Caesar (Acts 25:23–27).

Paul's testimony adheres to the traditional Roman courtroom. His exordium, or introductory address, included a polite, sincere, but not overly flattering, greeting to Agrippa (Acts 26:2–3). The narratio, or context of events, began with a description of Paul's childhood training as a Pharisee and time spent persecuting Christians before transitioning into his conversion to Christianity (Acts 26:4–18). Now, he begins his argumentio, or formal defense. He explains how Jesus gave him a commission in a "heavenly vision" which he faithfully fulfills by spreading the offer of forgiveness through repentance to the Jews and the Gentiles, an offer promised by Moses and the other Old Testament prophets (Acts 26:20–23).

The "heavenly vision" is likely a combination of Paul's initial meeting with Jesus on the road to Damascus (Acts 9:1–9), further instruction Paul received over the years directly following (Galatians 1:15–18), and Jesus' words Paul received while in a trance when he returned to Jerusalem (Acts 22:17–21). Jesus warned Paul that the Jews in Jerusalem would not accept his message; he needed to go out into the world and preach to the Gentiles. It is largely because of Paul's relationship with Gentile believers that he is on trial now (Acts 21:27–36).