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.

Acts 12:15 They said to her, "You are out of your mind!" But she kept insisting that it was so. They kept saying, "It is his angel."

Acts 12:17 But motioning to them with his hand to be silent, he described to them how the Lord had led him out of the prison. And he said, "Report these things to James and the brethren." Then he left and went to another place.

Peter after his Escape
'But he, beckoning unto them with the hand to hold their peace, declared unto them how the Lord had brought him forth out of the prison. And he said, Go shew these things unto James, and to the brethren, And he departed, and went into another place.' --ACTS xii. 17. When the angel 'departed from him,' Peter had to fall back on his own wits, and they served him well. He 'considered the thing,' and resolved to make for the house of Mary. He does not seem to have intended to remain there, so dangerously …
Alexander Maclaren—Expositions of Holy Scripture: The Acts

Delivered from Prison
"Now about that time Herod the king stretched forth his hands to vex certain of the church." The government of Judea was then in the hands of Herod Agrippa, subject to Claudius, the Roman emperor. Herod also held the position of tetrarch of Galilee. He was professedly a proselyte to the Jewish faith, and apparently very zealous in carrying out the ceremonies of the Jewish law. Desirous of obtaining the favor of the Jews, hoping thus to make secure his offices and honors, he proceeded to carry out …
Ellen Gould White—The Acts of the Apostles

How the Gospels came to be Written
[Illustration: (drop cap B) Early Christian Lamp] But how did the story of the Saviour's life on earth come to be written? We have seen that many years passed before any one thought of writing it down at all. The men and women who had really seen Him, who had listened to His voice, looked into His face, and who knew that He had conquered death and sin for evermore, could not sit down to write, for their hearts were all on fire to speak. But as the years passed, the number of those who had seen Christ …
Mildred Duff—The Bible in its Making