Now some time later, Agrippa the King and Bernice arrived at Caesarea on a welcoming visit to Festus. They stayed many days, and Festus showed Paul's case to the king, saying, "This is a man left prisoner by Felix; and when I was at Jerusalem, the chief priests and the elders of the Jews advised me about him, asking me to sentence him. I answered them that it was not the Roman custom to ever give any one up before the accused met the accusers face to face, and had an opportunity to defend themselves against the charges laid. When they therefore convened here, I made it a priority, and on the next day took my seat on the tribunal ordering the man to be brought in. When the accusers stood up, they brought no charge in his case of any evils I had anticipated; but had certain points of dispute with him about their own superstition and about one Jesus, who was dead, but whom Paul asserted to be alive. Being at a loss how to investigate these questions, I asked whether he wished to go to Jerusalem and be tried there. But Paul appealed to be kept in custody for the decision of the emperor, and so I had to command him to be held until I could send him to Caesar." And Agrippa said to Festus, "I should like to hear the man myself." "Tomorrow," said he, "you shall hear him." So on the morrow Agrippa and Bernice came with great pomp, entering the audience hall with the military tribunes and the prominent men of the city. Then, by command of Festus, Paul was brought in. Festus said, "King Agrippa and all who are present with us, you see this man about whom the whole Jewish people petitioned me, both at Jerusalem and here, shouting that he ought not to live any longer. But I found that he had done nothing deserving death; and as he himself appealed to the emperor, I decided to send him. But I have nothing definite to write to my lord about him. Therefore I have brought him before you, and, especially before you, King Agrippa, that, after we have examined him, I may have something to write. For it seems to me unreasonable, in sending a prisoner, not to indicate the charges against him."
When Luke wrote this account of Paul's appeal to Caesar, and its immediate aftermath, he had somehow gained access to the aide memoire of Festus. This gives added authenticity to Luke's work, even if it leaves us with other unanswered and even unanswerable questions. The book of Acts is the work of a historian who went to some lengths to collect the facts for his story. But when people read such historical parts of the Bible they too easily assume them to be the views of Christian writers who wrote one hundred years or more after the events described. Somehow the view is taken that such pre-modern writers were more concerned with their story than the truth of their data. That prejudice finds it extremely difficult to accept that this passage is an authentic historical record, based on primary sources. But is it naïve to accept this record as one based on authentic records and other eye-witness accounts? Clearly Luke had some access to official records. He transcribes what appears to be official notes about Paul's case composed by Festus and based upon records left behind by Felix. It seems highly likely that Luke's second book includes material used in Paul's defence in Rome. Whether this portion of his book is the counsel's "brief" we cannot know. But clearly it is germane to Paul's defence, and even if some of it came to light after his case was decided, it has a decisive impact upon how we understand the context in which Paul proclaimed his message.