Acts 12:25
And Barnabas and Saul returned from Jerusalem, when they had fulfilled their ministry, and took with them John, whose surname was Mark. And Barnabas and Saul returned from Jerusalem, when they had fulfilled their ministry, and took with them John, whose surname was Mark.
Barnabas and Saul returned from Jerusalem when they had fulfilled their ministry there, taking with them John whose surname was Mark. Mark joined them on their missionary journey, but when hardship and danger arose at Perga he became discouraged and went back to the comfort of his home in Jerusalem. His turning back was a failure of courage before hardship, not an abandonment of the Christian faith.