The martyrdom of James and the imprisonment of Peter (Acts 12:1-17) mark a pivotal moment in the life of the early church. Up until that time, the Twelve had remained in Jerusalem while Philip and other evangelists scattered about the region preaching the gospel. Now, however, even the Twelve will begin to disperse.
Peter, following his miraculous escape from prison, was obviously a wanted man. He would not be able to remain in Jerusalem, thus creating a leadership vacuum. But Peter already had someone in mind to fill the void. After explaining how an angel of the Lord delivered him from prison, he directed those gathered at the home of Mary, the mother of Mark, to "Tell these things to James and to the other brothers."
Acts 15), it is James who presides.
The transition that took place as Peter handed off leadership to James preserved a unique place for the Jerusalem church. While the mission of the church became focused more on the Gentile world, the church in Jerusalem would remain largely a Jewish congregation, even maintaining many of the Jewish distinctions, right down to the time of Jerusalem's demise in A.D. 70. It stood as a reminder to the increasingly evangelized Gentile world that salvation was a gift of God given first through the people of Israel.