Paul's question put King Agrippa in a difficult position. If Agrippa said he believed the prophets, Paul would press the Christian message. If Agrippa denied belief in the prophets, he would upset the devout Jews who were listening.
Agrippa knew Paul was not crazy and that Paul's testimony about Jesus was historically accurate (26:26). So, Agrippa avoided answering Paul's question and refused to consider the claims of Christ. Agrippa said that Paul's statement was too short for him to make a responsible decision.