John Chrysostom

Festus saw the boldness, and what says he? For Paul was all along addressing himself to the king-he was in a manner annoyed, and says to him, "Thou art beside thyself, Paul:" for, "while he thus discoursed, Festus said with a loud voice, Paul, thou art beside thyself: much learning doth make thee mad." What then says Paul? With gentleness, "I am not mad," says he, "most noble Festus; but speak forth the words of truth and soberness."

Homily on Acts 52 ↗

Theophylact of Ohrid

Since Paul constantly directed his speech to the king, Festus, being somewhat offended and seeing Paul's boldness, said out of vexation and anger: "You are out of your mind, Paul!" Paul then explains to him the reason why he directed his word to the king.

Commentary on Acts ↗

Oecumenius

Since he was always conversing with the king, as if Festus had suffered something, and seeing the boldness, from anger and outrage, he says, "You are mad."

Commentary on Acts ↗

Bede

Festus said in a loud voice: You are insane, Paul. He deemed it insanity that a man in chains, instead of just defending himself from external accusations, made a speech about the conscience by which he gloried inside and reviewed all the miracles of the revelation by which he was transformed from a persecutor into an apostle, and discussed the dispensation of our Redeemer and argued vehemently about the resurrection of the dead. …

Commentary on Acts ↗