Festus said with a loud voice - Amazed at the zeal of Paul. Paul doubtless evinced deep interest in the subject, and great earnestness in the delivery of his defense.
Thou art beside thyself - Thou art deranged; thou art insane. The reasons why Festus thought Paul mad were, probably:
(1) His great earnestness and excitement on the subject.
(2) His laying such stress on the gospel of the despised Jesus of Nazareth, as if it were a matter of infinite moment. Festus despised it; and he regarded it as proof of derangement that so much importance was attached to it.
(3) Festus regarded, probably, the whole story of the vision that Paul said had appeared to him as the effect of an inflamed and excited imagination, and as a proof of delirium. This is not an uncommon charge against those who are Christians, and especially when they evince unusual zeal. Sinners regard them as under the influence of delirium and fanaticism; as terrified by imaginary and superstitious fears; or as misguided by fanatical leaders. Husbands often thus think their wives to be deranged, and parents perceive their children that, and wicked people assume the ministers of the gospel to be crazy. The frivolous think it proof of derangement that others are serious, anxious, and prayerful; the rich, that others are willing to part with their property to do good; the ambitious and worldly, that others are willing to leave their country and home to go among the Gentiles to spend their lives in making known the unsearchable riches of Christ. The really sober and rational part of the world they who fear God and keep his commandments; they who believe that eternity is before them, and who strive to live for it - are thus charged with insanity by those who are really deluded, and who are thus living lives of madness and folly. The tenants of a madhouse often think all others deranged but themselves; but there is no madness so great, no delirium so awful, as to neglect the eternal interest of the soul for the sake of the pleasures and honors which this life can give.
Much learning - It is probable that Festus was acquainted with the fact that Paul was a learned man. Paul had not, while before him, manifested particularly his learning. But Festus, acquainted in some way with the fact that he was well-educated, supposed that his brain had been turned, and that the effect of it was seen by devotion to a fanatical form of religion. The tendency of long-continued and intense application to produce mental derangement is everywhere known.
Doth make thee mad - Impels, drives, or excites thee περιτρέπει peritrepei to madness.