Acts 12:7

The angel of the Lord, whose office it is to minister for the heirs of salvation, Hebrews 1:14, and who willingly fulfil this will of the Lord. Came upon him, as Luke 2:9, suddenly and unexpectedly. A light shined in the prison; whether this light was from the bright body the angel assumed, or from some other cause, we are not told, and therefore it is not necessary for us to know; but it was a light only to Peter, but darkness to his keepers; as the pillar of fire enlightened only the Israelites; which made them both the more strange and miraculous. The angel smote Peter (as one jogs, or gently strikes another) to awaken him; thus God was waking, though Peter slept; and by his providence watches over all his people for their preservation. His chains fell off from his hands; chains could not hold any whom God will have free; every thing loses its force when God suspends or withdraws his concurrence.