What does Acts 26:28 mean?

Paul has struck a nerve. Governor Festus has invited him to present his defense before King Agrippa II, Bernice, and the leaders of Caesarea Maritima (Acts 25:23). Paul knows Agrippa understands Jewish culture and religion (Acts 26:2–3). Further, he believes Agrippa believes—in some sense—the prophecies recorded in Jewish Scriptures (Acts 26:27). Agrippa knows the story of how Jesus of Nazareth reportedly rose from the dead, even though he was probably only about six years old at the time. He understands that Jesus-worship is a sect of Judaism, and that they're called "Christians." Unlike Festus, who thinks Paul has gone insane (Acts 26:24), Agrippa acknowledges the logic of Paul's argument and can't counter it.

When Paul challenges Agrippa to believe in Jesus, he deflects. He is a king: the last king of the Jews. To confess belief in Jesus as the Jewish Messiah and Son of God would be political disaster. Further, he's surrounded by Roman leaders who call Caesar son of the gods and likely don't believe in the physical resurrection of the dead.

Agrippa cannot follow Paul's Christ, but neither can he condemn Paul for leading an illegal cult (Acts 24:5). Festus called him to listen to Paul to see if he had committed any crime. Agrippa has no choice to but to declare that he hasn't. In fact, he says, "This man could have been set free if he had not appealed to Caesar" (Acts 26:32).