Festus had admitted to King Agrippa and those present with him that he presumed Paul was innocent, but because he appealed to Caesar rather than go to Jerusalem to be tried by his accusers, he (i.e. Festus) had nothing to write to the Emperor by way of explanation of the reason for his imprisonment and need of judgment. (Acts 25:24-27).
Paul began his defense by recognizing King Agrippa’s familiarity with the Jewish faith and that he (Paul) could speak more freely, knowing his words would be understood. Paul mentioned, as he had done before the Jews who laid hands on him in the Temple (Acts 22), that at one time he had persecuted the very Way which he now followed. He told the king that he had seen a vision while on his way to Damascus. This vision was a light that was so bright both Paul and the men with him fell to the ground. There it was that he both heard and saw Jesus alive.
Paul made no mention before Agrippa that the men with him did not understand the voice (sound) they heard, nor does he tell of his blindness due to the bright light and his subsequent healing at the word of Ananias, a brother in the Lord. At this point, either Paul or Luke merges two of Paul’s visions, the one he had of the Lord on the way to Damascus, and another, concerning which he told the Jews that he had later in the Temple (Acts 22:17-21).
In any event his testimony before King Agrippa in Festus’s court is a summary of two or more visions Paul had of the Lord. What he tells Agrippa is that he met Jesus and was told he was chosen by him to bring the very message to the gentiles that Moses and the Prophets, themselves, foretold, so that the gentiles with the Jews could have an opportunity to be forgiven of their sins and together with the Jews obtain an inheritance from God.
Paul says he was not disobedient to the vision and command he received of the Lord, but went from Damascus to Jerusalem and from there, throughout the coasts of Judea and then to the gentile cities throughout much of the empire. He preached Christ and called for repentance and good works proving repentance—and the very fact that he had come to Jerusalem after several years absence with a gift from the gentiles to the poor at Jerusalem (Acts 24:17) testifies of the truth of his words here.
At this point Festus interrupted Paul, saying that much learning had affected his common sense and was no longer speaking credibly (Acts 26:24). Whereupon, Paul claimed he was speaking very sensibly according to the Jewish faith which King Agrippa knew very well, for Paul had been speaking freely, knowing his manner would be understood by him. The issues of what Jesus said and did and what had been done to him and preached afterward were not hidden or kept secret. Paul was convinced the King both knew what had occurred and how it all fit together according to Moses (the Law) and the Prophets.
King Agrippa was perhaps a little embarrassed for everyone present to hear Paul say this, for he asked Paul before everyone, if, with such a short testimony, he expected the king to become a Christian as well (Acts 26:28). Nevertheless, when the king conferred with the governor privately, he admitted that Paul should have been set free, had it not been for the fact Paul had exercised his right as a Roman citizen to be heard before Caesar.