John Calvin Commentary Acts 12:10

"And when they were past the first and the second guard, they came unto the iron gate that leadeth into the city; which opened to them of its own accord: and they went out, and passed on through one street; and straightway the angel departed from him." — Acts 12:10 (ASV)

When they were past. God could have carried Peter away in an instant; but He overcomes various difficulties one after another, so that the glory of the miracle might be greater. So He created the world in six days (Genesis 1), not because He had any need of time, but so that He might better help us to meditate upon His works (Exodus 20:11), for He applies His manner of doing to our capacity and to the increase of faith. If Peter had suddenly been carried to the house where the brethren were assembled, then only one deliverance would have been acknowledged; but now we see, as it were with our eyes, that he was delivered more than ten times.