King Agrippa - This bold personal address is an instance of Paul's happy manner of appeal. He does it to bring in the testimony of Agrippa to meet the charge of Festus that he was deranged.
Believest thou the prophets? - The prophecies respecting the character, the sufferings, and the death of the Messiah.
I know that thou believest - Agrippa was a Jew; and, as such, he of course believed the prophets. Perhaps, too, from what Paul knew of his personal character, he might confidently affirm that he professed to be a believer. Instead, therefore, of waiting for his answer, Paul anticipated it, and said that he knew that Agrippa professed to believe all these prophecies respecting the Messiah. His design is evident. It is:
(1) To meet the charge of derangement, and to bring in the testimony of Agrippa, who well understood the subject, to the importance and the truth of what he was saying.
(2) to press on the conscience of his royal hearer the evidence of the Christian religion, and to secure, if possible, his conversion. "Since thou believest the prophecies, and since I have shown that they are fulfilled in Jesus of Nazareth; that he corresponds in person, character, and work, with the prophets, it follows that his religion is true." Paul lost no opportunity in pressing the truth on every class of people. He had such a conviction of the truth of Christianity that he was deterred by no rank, station, or office; by no fear of the rich, the great, and the learned; but everywhere urged the evidence of that religion as indisputable. In this lay the secret of no small part of his success. A man who really believes the truth will be ready to defend it. A man who truly loves religion will not be ashamed of it anywhere.

Believest thou the prophets? - Having made his elegant compliment and vindication to Festus, he turns to Agrippa; and, with this strong appeal to his religious feeling, says, Believest thou the prophets? and immediately anticipates his reply, and, with great address, speaks for him, I know that thou believest. The inference from this belief necessarily was: "As thou believest the prophets, and I have proved that the prophets have spoken about Christ, as suffering and, triumphing over death, and that all they say of the Messiah has been fulfilled in Jesus of Nazareth, then thou must acknowledge that my doctrine is true."

(9) King Agrippa, believest thou the prophets? I know that thou believest.
(9) Paul, as it were forgetting that he stood a prisoner to defend his cause, does not forget the office of his apostleship.

King Agrippa, believest thou the prophets?.... What they have said concerning the person, office, sufferings, death, and resurrection of Christ, and that what they have said is fulfilled in Jesus of Nazareth?
I know that thou believest; that what the prophets said were true, and are accomplished.

believest thou the prophets? I know that thou believest--The courage and confidence here shown proceeded from a vivid persuasion of Agrippa's knowledge of the facts and faith in the predictions which they verified; and the king's reply is the highest testimony to the correctness of these presumptions and the immense power of such bold yet courteous appeals to conscience.

King Agrippa, believest thou the prophets? The king professed to believe them. Yet those very prophets, as Paul had shown, testified to all the facts of the career of Jesus of Nazareth and his claims to Messiahship. This personal appeal deeply moved the king, as his reply shows.
Almost thou persuadest me to be a Christian. The Revision changes the translation somewhat, but I have little doubt but that the Common Version gives the idea. The king, like Felix (Acts 24:25), was deeply moved; the fact that he and Festus decided (Acts 26:32) that Paul was not a transgressors show that they were favorably impressed; it was no occasion for an ironical answer, and Paul took the remark as in earnest, and added still another appeal. Chrysostom, Luther, Beza, Bengel and Howson take this view.
Paul said, I would to God, etc. His reply is courteous, but of intense earnestness, a last effort to save souls that were deeply stirred. He would that king and governor, all, Jew and Gentile, shared his hope of a glorious inheritance, and were, like himself, at peace with God;--such as he, save these bonds. It is probable that his chains were then hanging upon his arms, and that he indicated them by a gesture.

King Agrippa, believest thou the prophets? - He that believes these, believes Paul, yea, and Christ. The apostle now comes close to his heart. What did Agrippa feel when he heard this? I know that thou believest! - Here Paul lays so fast hold on the king that he can scarce make any resistance.