What does Acts 26:18 mean?

Governor Festus, King Agrippa II, Agrippa's sister/lover Bernice, and the military and civil leadership of Caesarea Maritima are listening to Paul. He is defending himself against charges by the Sanhedrin that he started riots, desecrated the temple, and leads a cult not authorized by the Roman government (Acts 24:5–6). But because the background of his defense requires an account of his conversion from church persecutor to Jesus-follower, he easily slips in a gospel message.

Here, Paul may be paraphrasing Jesus' overall message to him, given in the first years after he became a Christian. Or he may be recounting something Jesus told him directly which isn't recorded in the original story in Acts 9:3–19; it's probably the former. Jesus chose Paul to tell Gentiles about Him (Acts 26:16); this is His message.

Isaiah prophesied Paul's message. He said of the Messiah, "I will give you as a covenant for the people, a light for the nations, to open the eyes that are blind, to bring out the prisoners from the dungeon, from the prison those who sit in darkness" (Isaiah 42:6–7). And, later, "It is too light a thing that you should be my servant to raise up the tribes of Jacob and to bring back the preserved of Israel; I will make you as a light for the nations, that my salvation may reach to the end of the earth" (Isaiah 49:6). During His earthly ministry, Jesus explained that He is the light (John 8:12; 12:46).

Although Festus called this meeting, Paul is directing his words to Agrippa (Acts 26:2–3). Agrippa is the great-grandson of Herod the Great (Matthew 2:1–4) and the son of Agrippa I. Ethnically, he is a combination of Jewish, Idumean, and Nabatean Arab, among others. Paul will go on to ask him, "King Agrippa, do you believe the prophets? I know that you believe" (Acts 26:27). "Idumean" is the New Testament-era term for "Edomite." Agrippa is, in part, descended from the nation that Jacob's brother Esau ruled (Genesis 36:1). The thought that an Edomite king could regain part of the spiritual inheritance which Esau sold (Genesis 25:29–34) startles Agrippa. "In a short time would you persuade me to be a Christian?" he asks Paul. Paul affirms, "Whether short or long, I would to God that not only you but also all who hear me this day might become such as I am—except for these chains" (Acts 26:28–29).

Paul offers Agrippa everything: the lost inheritance, freedom from the power of Satan, even forgiveness from the incest Agrippa commits with his sister (1 Corinthians 6:9–11). There's no historical evidence Agrippa takes the offer.