What does Acts 26:25 mean?
Governor Festus thinks Paul is crazy. Paul has been explaining how his life changed when he met Jesus. He had been raised as a strict Pharisee and violently persecuted the church. But when he met Jesus, he applied that same fervor to the propagation of Jesus' message of forgiveness of sins and reconciliation with God. Paul also alluded to the prophecy of the Messiah given in the Jewish Scriptures, knowing that his other primary audience member, King Agrippa II, would understand (Acts 26:2–24).Festus does not understand. Though he is a fair ruler and wants good relationships with Jewish leadership, he has only been in the territory for a few weeks. He doesn't understand the Jewish religion or culture. If he thinks anything about Christianity, he perceives it as a sect of Judaism, one which the Sanhedrin hates. None of what Paul is saying makes sense to him.
"Rational" is from the Greek root word sōphrosynē which means "soundness of mind, or sobriety." Paul is not insane; he is speaking logically. "True" is from the Greek root word alētheia. It can refer to objective or subjective truth, but also refers to the accurate view of God, along with humanity's responsibility to Him as Creator and Lord.
Agrippa understands what Paul is saying, at least intellectually. He can see how Jesus fulfills Old Testament prophecy. He understands his responsibility to God. But he doesn't want it. He's in an incestuous relationship with his sister. He's from a long line of rulers who understand Judaism and follow it superficially. Yet this family line unanimously refuses to submit to God. God promised Paul would share Jesus' message to kings, but He never said the kings would accept (Acts 9:15).