New Defender's Study Bible Notes
12:9 wist. That is, “know.”
12:12 many were gathered together. This was most likely the same upper room where they had been praying before the coming of the Holy Spirit on the day of Pentecost (Acts 1:13), and where they had observed the last supper with the Lord (Luke 22:12). Mark had probably been in the house with them both times.
12:15 Thou art mad. In spite of the fact that they had been praying without ceasing for Peter’s release (Acts 12:5), they at first could not believe that God had answered their prayers!
12:15 It is his angel. There are, indeed, “guardian angels” assigned to believers (e.g., Psalm 34:7; Matthew 18:10; Hebrews 1:14), and it was evidently believed that each such angel could, if appropriate, assume the appearance of his particular charge. There is no Scriptural basis anywhere for the pagan belief that those who die still linger as ghosts. Besides, the Christians knew that Peter was not scheduled for execution until after the Passover (Acts 12:4), so there is no reason to think that, by “his angel,” they meant “his spirit.”