12 When he realized this, he went to the house of Mary, the mother of John whose other name was Mark, where many were gathered together and were praying. 13 And when he knocked at the door of the gateway, a servant girl named Rhoda came to answer. 14 Recognizing Peter's voice, in her joy she did not open the gate but ran in and reported that Peter was standing at the gate. 15 They said to her, “You are out of your mind.” But she kept insisting that it was so, and they kept saying, “It is his angel!” 16 But Peter continued knocking, and when they opened, they saw him and were amazed. 17 But motioning to them with his hand to be silent, he described to them how the Lord had brought him out of the prison. And he said, “Tell these things to James and to the brothers.”  Then he departed and went to another place.
18 Now when day came, there was no little disturbance among the soldiers over what had become of Peter. 19 And after Herod searched for him and did not find him, he examined the sentries and ordered that they should be put to death. Then he went down from Judea to Caesarea and spent time there. (Acts 12:11-19 (ESV)
The introduction of John Mark, or as most of us know him, Mark, the evangelist. This is where the Christians of Jerusalem had been holding a prayer vigil for Peter. There prayers had been answered and they could hardly believe it. They think it is Peter’s angel, not Peter himself. In any case, Peter doesn’t stay long but leaves for another place. It doesn’t say where, many believe that it was at this time that Mark and Peter first went to Rome. Mark also worked with Paul, Barnabus and Luke. But for the most part Mark was Peter’s administrative assistant. His gospel was written at the feet of Peter who considered it his gospel.
Before Peter leaves he tells the congregation to let James and the brothers know. These would be the brothers of Jesus who had come to believe in the resurrection and despite having tried to kill Jesus during his life, had come to have respected positions in the Early Church of Jerusalem. James would be the first post apostolic bishop of Jerusalem. Whether he was the one who wrote the book we know of as James is up for considerable debate.
Herod isn’t happy, orders the death of the soldiers who were to guard Peter and heads to Caeserea, where he can live as the Roman he is.