John Calvin Commentary Acts 16:30

"and brought them out and said, Sirs, what must I do to be saved?" — Acts 16:30 (ASV)

Sirs, what must I do? He asks for counsel in such a way that he thereby shows his willingness to be obedient. From this we see that he was profoundly affected, so that he was ready to do whatever they might command him, whom only a few hours before he had bound harshly.

The wicked often, when they see wonders, though they may tremble for a time, yet are soon after made more obstinate, as happened to Pharaoh (Exodus 8:8, 32). At least, they are not so subdued that they surrender themselves to God.

But in this case, the jailer (acknowledging the power of God) was not merely afraid for a moment, only to return immediately to his former cruelty; instead, he shows himself obedient to God and desirous of sound and wholesome doctrine. He asks how he may obtain salvation. This shows more plainly that he was not suddenly overcome by some slight fear of God alone, but was truly humbled enough to offer himself as a student to God's ministers.

He knew that they had been put in prison for no other reason than that they were challenging the established religious order. Now he is ready to hear their doctrine, which he had previously despised.