John Calvin Commentary Acts 16:34

"And he brought them up into his house, and set food before them, and rejoiced greatly, with all his house, having believed in God." — Acts 16:34 (ASV)

He rejoices that he believed. The external profession of faith was previously commended in the jailer; now the inward fruit of it is described. When he lodged the apostles and was not afraid of punishment, but courteously entertained them in his own house, contrary to what he was ordered by the magistrate, he testified that his faith was not idle.

And that joy of which Luke speaks in this place is a unique good, which every person has from their faith. There is no greater torment than an evil conscience; for the unbelievers, though they seek by all means to bring themselves into a certain stupor, yet because they have no peace with God, they must necessarily quake and tremble.

But even if they do not perceive their present torments—indeed, they may rage and act like madmen through frenzied and unbridled licentiousness—yet they are never quiet, nor do they enjoy quiet joy. Therefore, sincere, quiet, and stable joy proceeds from faith alone, when we perceive that God is merciful to us.

In this respect, Zacharias says, “Rejoice and be glad, O daughter of Sion, behold, thy King cometh.” Indeed, this effect is everywhere in Scripture attributed to faith: that it makes souls joyful. Therefore, let us know that faith is not a vain or dead imagination. Instead, it is a living seal [an inward sense] of the grace of God, which brings perfect joy because of the certainty of salvation—a joy of which it is fitting that the wicked are devoid, as they both flee from the God of peace and disturb all righteousness.