John Calvin Commentary Acts 26:25

"But Paul saith, I am not mad, most excellent Festus; but speak forth words of truth and soberness." — Acts 26:25 (ASV)

I am not mad. Paul is not angry, nor does he sharply reprimand Festus for his blasphemous speech; indeed, he speaks to him with great submission. For it was not the place for a reprimand, and it was fitting for him to pardon the man's ignorance, since he did not directly oppose God. Also, he had respect for his person [office]. For though he was unworthy of honor, yet he was in authority. And yet, despite that, he does not therefore yield to his blasphemy, but he defends the glory of the word of God. From this we also see that, not caring for himself, he was only concerned about his doctrine. For he does not boast of his intellect; he does not strive in defense of his own wisdom; but he is content with this defense alone: that he teaches nothing but what is true and sober.

Furthermore, [the] truth is set against all kinds [of] fallacies and fraud; sobriety against all kinds [of] frivolous speculations and thorny subtleties, which are only seeds of contention. Paul does, indeed, refute Festus’ error; yet from this we may gather what the best manner of teaching is: namely, that which is not only free from all fallacies and deceit, but also does not make the minds of men drunk with vain questions, and does not nourish foolish curiosity, nor an intemperate desire to know more than is proper, but is moderate and good for sound edification.