John Chrysostom
The earthquake was such that the guard awoke. The doors were opened in such a way that one could not help marveling at what had happened.
But the prisoners did not see this; otherwise they all would have fled. Why was this sign given, and not some other?
Because it was especially able to instruct and persuade him. Had it not happened, he would have been in danger. Miracles do not instruct us so much as what concerns our salvation.
And so that the earthquake might not seem to have happened by itself, what followed occurred as a testimony to the miracle. It took place at night, because the Apostles did nothing out of vainglory, but everything for salvation.
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What happened afterward? Something utterly extraordinary and wondrous. The bonds were loosed, and those who had been bound bound the unbound.
By their nature, bonds hold the bound man fast and do not give him freedom. But here the unbound jailer came and fell at the feet of the bound Paul.
Material bonds are of such a nature that they hold the one bound. But the bonds of Christ have such power that they subject the unbound to the bound.
Why, then, did God shake the prison? He wished to awaken the jailer's attention to what was happening. He loosed the bodily bonds of the bound, in order to loose the spiritual bonds of the jailer. See verse 29.