In this version of the story Paul emphasizes the fact that God called him to be the “light to the gentiles” (Acts 26:16-18). This commission is based on Isaiah 49:6. Paul seems to have always conceived of his mission through the lens of this text, which is somewhat unique in first century Judaism. The text appears as a part of the “servant texts” in Isaiah. This series of four prophecies describes the “Servant of the Lord” whose suffering brings about some kind of salvation for Israel.
There seems to have been some discussion of who the servant was; recall that the Ethiopian Eunuch was reading this text in Acts 8 and the idea of a suffering, dying and rising messiah appears at several points in the book of Acts. This is anticipated as early as Luke 2:32, Simeon’s blessing on Jesus cites Isaiah and proclaims that this salvation has come to Israel.
Paul offers a somewhat different view – the Servant who suffered is Jesus (Luke 4:18, Is 61:1), but the “Light to the Gentiles” is applied to Paul and his ministry. Turning to God and “opening eyes” is likely a reference back to the paradigmatic miracle on Cypress, the blinding of Bar-Jesus. Like Isaiah, both Jesus and Paul ministered to blind people, people who were in need of healing in their understanding, so that they might believe that Jesus is in fact the Messiah. Paul is sent to preach repentance to both the Jews and the Gentiles (recalling Romans 1:16-17, to the Jew first).
Festus interrupts Paul’s speech: “You are out of your mind!” It is possible that this means that Paul’s knowledge of esoteric doctrines find things that are not necessarily true. This may reflect the common-sense “down to earth” Roman worldview. Festus is saying that the conclusions to which Paul comes is “beyond common sense,” not that these are strange and outlandish things.
Paul states that he is speaking “true and rational words.” This description is good Greek rhetoric, sobriety is a chief virtue in Greek philosophy. Agrippa, on the other hand, understands that Paul’s speech has a persuasive value, that he is trying to convince them both of the truth of the Gospel. What Paul has done has “not been done in a corner,” but rather out in the open for all to hear and evaluate. This too is a feature of good philosophy and rhetoric, those who engage in secrets and mysteries are questionable (and probably not sober and self-controlled).