Albert Barnes 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)

But the angel of the Lord. This does not denote any particular angel, but simply an angel. The article is not used in the original. The word angel denotes, properly, a messenger, and particularly it is applied to the pure spirits that are sent to this world on errands of mercy (See Barnes on Matthew 1:20).

The case here was evidently a miracle. An angel was employed for this special purpose; and the design might have been:

1. to reprove the Jewish rulers, and to convince them of their guilt in resisting the gospel of God;
2. to convince the apostles more firmly of the protection and approval of God;
3. to encourage them more and more in their work, and in the faithful discharge of their high duty; and
4. to give the people a new and impressive proof of the truth of the message which they brought. That they were imprisoned would be known to the people. That they were made as secure as possible was also known. When, therefore, the next morning, before they could have been tried or acquitted, they were found again in the temple, delivering the same message still, it was a new and striking proof that they were sent by God.