‘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. And behold, an angel (or ‘messenger') of the Lord stood by him, and a light shone in the cell, and he smote Peter on the side, and awoke him, saying, “Rise up quickly.” And his chains fell off from his hands.'
The night arrived prior to the day when Peter was to be brought out, the night following the Sabbath of the seventh day of unleavened bread. It was the time when ‘Herod was about to bring him forth.' Peter was asleep between two soldiers, bound with two chains, while a further two soldiers watched the door. They were determined to keep him safe. And as for Peter, there was no tremor on his brow. He was sleeping like a babe (compare Psalms 3:5; Luke 8:23). He was ready to go to meet his Lord. (Or he may simply have always been a very heavy sleeper. But Luke does go to great pains to stress how heavily he had been sleeping so that it took him a good while to wake up properly).
But then all heaven broke loose. He found himself rudely awakened by a blow to his side, and saw a light shining in the cell, and found there ‘the angel (messenger) of the Lord' who urged him to rise up quickly. And when he sought to do so the chains fell off his hands.
‘The chains fell off his hands.' Whether as a result of being unlocked or simply by a move of the visitant's hand is unimportant. What was important was the ease with which God disposed of them. All the king's efforts were in vain. They were as chaff before the Almighty's wind.
‘The angel of the Lord.' This could mean ‘the messenger of the Lord' and be referring to a human agency. On the other hand in Acts 12:23 the angel of the Lord is undoubtedly an angelic agency, and the same applies in Acts 8:26 (as is also the case in the Old Testament). Thus the most natural meaning here would be of specific divine intervention.