John Calvin Commentary Acts 12:6

"And when Herod was about to bring him forth, the same night Peter was sleeping between two soldiers, bound with two chains: and guards before the door kept the prison." — Acts 12:6 (ASV)

When he was about to bring him forth. It seems at first glance that the Church prays to little purpose, for the day was now appointed when Peter should be put to death, and he is within one night of death. Yet the faithful do not cease to pray, because they know that when the Lord purposes to deliver His own, He often takes His time until the utmost point of necessity, and that He has various ways to deliver. Secondly, we may think that they did not so much pray for Peter’s life, as that the Lord would arm him with invincible fortitude for the glory of the gospel, and that God would not expose the gospel of His Son to the reproaches and slanders of the wicked.

That night he slept. All these circumstances further demonstrate the wonderful power of God. For who would not have thought that Peter was already swallowed up by death? Although he still drew breath, he had no opening through which to escape, since he was surrounded by many threats of death.

Therefore, his escape from the midst of such perils, his safe passage among his executioners, the melting and loosening of the chains, and the iron gate opening by itself for him—all these things clearly show that it was a purely divine deliverance. It was also beneficial for Peter to be taught in this way by these signs, so that he might with greater assurance immediately declare to people the grace of God thus made known.

Furthermore, it is apparent from this strict imprisonment that Herod was utterly determined that Peter would not go away alive.