Festus had already examined Paul and found nothing to condemn. King Agrippa and Bernice arrived in Caesarea, so Festus gave them a brief update on his prisoner, Paul, before they examined him. In his summary of Paul's previous trial, Festus said, "When the accusers stood up, they began bringing charges against him not of such crimes as I was expecting, but they simply had some points of disagreement with him about their own religion and about a dead man, Jesus, whom Paul asserted to be alive" (Acts 25:18-19).
To Festus, the argument between Paul and the Jewish leaders was all about a dead man, Jesus. The Jews, of course, claimed Jesus was dead. They had paid the guards who had been at Jesus' tomb at the time of His resurrection to spread a lie: Jesus' body was stolen from the tomb by His disciples (Matt. 28:12-15). And so it was their word against Paul's. Paul claimed this Jesus had been raised from the dead and now alive! It's obvious Festus wasn't buying that story.
This illustrates 1 Cor. 1:23-24: "But we preach Christ crucified, to Jews a stumbling block and to Gentiles foolishness, but to those who are the called, both Jews and Greeks, Christ the power of God and the wisdom of God." If you don't want to believe in Jesus Christ, it's easy. Simply tell yourself there is no such thing as a person raised from the dead, and you will have sufficiently destroyed the basis for Christianity in your own mind. However, if you are willing to let the evidence speak for itself, Christ is the power of God and the wisdom of God. He's not a dead man, He's the living Son of God.
Nathan