John Calvin Commentary Acts 5:19

"But an angel of the Lord by night opened the prison doors, and brought them out, and said," — Acts 5:19 (ASV)

The angel of the Lord. The Lord brought the apostles out of prison, not because he intended to deliver them completely from the hands of their enemies (for he allowed them afterwards to be brought back and beaten with rods), but he intended to declare by this miracle that they were in his hand and under his protection.

He did this so that he might maintain the credibility of the gospel, partly so that the Church might receive additional confirmation through it, and partly so that the wicked might be left without excuse.

Therefore, we must not always hope—indeed, not always desire—that God will deliver us from death. Instead, we must be content with this one thing: that our life is defended by his hand, as far as is fitting.

In using the ministry of an angel, he acts according to his usual custom, for he testifies everywhere in the Scriptures that the angels are ministers of his goodness towards us. Nor is this an empty speculation, for it is a beneficial help for our weakness to know that not only does God care for us, but also that the heavenly spirits watch for our safety.

Again, it was a significant pledge of God’s love towards us that the noblest of all creatures are appointed to ensure our safety. The angel opened the prison in the night, because he chose not to perform the miracle where the wicked could see him, although God intended for the accomplished miracle to be known through the event itself.