John Chrysostom
Then Festus, seeing with what boldness he spoke to the king and did not address him as though he had suffered some offense, said: "Are you mad, Paul?"
Listen to what follows, to see that he said this from irritation: "As he thus made his defense, Festus said with a loud voice, 'You are mad, Paul! Much learning is driving you mad.'"
Thus it was a voice of indignation and anger.
Paul preaches the truth, but Festus objects to him: "You are mad, Paul! Much learning is driving you mad." What is full of sound reason seems madness to those who cannot receive it.