John Calvin Commentary Acts 12:17

"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, Tell these things unto James, and to the brethren. And he departed, and went to another place." — Acts 12:17 (ASV)

Tell James and the brethren: By "brethren" I understand not everyone who was of the Church but the apostles and elders. For though it was necessary that the miracle should be made known to all, yet Peter, for honor's sake, would worthily have his colleagues in office informed of it.

Ecclesiastical writers after Eusebius report that this James was one of the disciples; but since Paul counts him among the three pillars of the Church (Galatians 2:9), I do not think that a disciple was advanced to that dignity, with the apostles being set aside. Therefore, I rather conjecture that this was James the son of Alpheus, whose holiness was such that it caused the Jews to wonder at it.

And there are two reasons why Peter would have this joyful message brought to the brethren: namely, that he might rid them of the worry that troubled them; and secondly, that they might be encouraged by such an example of God’s goodness to be more bold.

Regarding his going to another place, I think it was done for this reason: since the house was well known and famous, and because many of the brethren gathered there, he could hide elsewhere with less danger. Therefore, he sought a place less suspected by the enemy, so that he might not only save himself but also his hostess and others.