Adam Clarke on Acts 12:15

It is his angel - It was a common opinion among the Jews that every man has a guardian angel, and in the popish Church it is an article of faith. The Jews also believed that angels often assumed the likeness of particular persons. They have many stories of the appearance of Elijah in the likeness of different rabbins. As αγγελος signifies in general a messenger, whether Divine or human, some have thought that the angel or messenger here means a servant or person which the disciples supposed was sent from Peter to announce something of importance to the brethren: it was also an opinion among the Jews, even in the time of the apostles, as appears from Philo, that the departed souls of good men officiated as ministering angels; and it is possible that the disciples at Mary's house might suppose that Peter had been murdered in the prison; and that his spirit was now come to announce this event, or give some particular warning to the Church.