But the word of God grew and multiplied. And Barnabas and Saul returned from Jerusalem when they had fulfilled their mission, bringing with them John whose other name was Mark.
The word "from" has been replaced in some very ancient manuscripts by the word "to" as in "to Jerusalem". If "to" is right then Saul and Barnabas, with Mark, had a mission somewhere else. That does not square with what Luke has written and it is most likely a copyist's error caused by an assumption in the earliest days of the church that all movement revolved around Jerusalem. In fact it didn't and Luke seems to be going to some lengths to explain why. Often the Christian mission has been viewed as from Jerusalem to other centres. In such a view it is not easy to understand how other places had a mission to Jerusalem. But Luke is telling us just that. As God's Spirit was poured out so new centres of activity developed from which, under the same Spirit's guidance, new ventures and missions, could be undertaken. The "mission" of Saul and Barnabas consisted in coming up to Jerusalem from Antioch, bringing with them the resources collected to help their fellow believers in that time of famine. At the end of Chapter 11 Luke tells us they did this in response to the prophecy of Agabus; at the end of Chapter 12, their return to Antioch is noted. It is fair to assume that during their time in Jerusalem, Agrippa murdered James and arrested Peter. So it is not far-fetched to suggest that they were present when Peter knocked on the door of the house of Mark's mother after the angel had engineered his escape from prison. John Mark was the nephew of Barnabas, a Levite from Cyprus. Luke seems to be indicating to his readers how his two books are related to the gospel written by Mark. The believers in Antioch, Jew and Gentile, stood in need of well-documented eye-witness accounts of Jesus' ministry among His disciples. And so did Barnabas and Saul. Mark was very useful to them in a number of ways.
When Luke says "they returned from Jerusalem", it seems Luke is giving us the "Antioch point of view". He then recounts how the church there commissioned Saul and Barnabas to take the good news throughout that Gentile region. Luke has also just recounted how the Antioch church supported the Christians in Judea. We also have heard of Peter's disciplined explanation of his attitude to Gentile believers after the Holy Spirit's outpouring at the house of Cornelius.
Later, when Saul had changed his name to Paul, he and Barnabas returned to Jerusalem to address the circumcision controversy in their churches. It seems clear that Paul was referring to Acts 15 in Galatians 2:9-10, but it is possible that as they left Jerusalem for Antioch they had even then been sent with apostolic approval for their mission to the Gentiles. Maybe they had briefly met Peter in Mark's mother's house that night.
It was the Holy Spirit's guidance which helped Luke write his books. Mark's Gospel was also an important documents for the ongoing work.