Acts 13 John Mark goes with Paul and Barnabas on a missionary trip to Cyprus where they face a standoff with a hostile sorcerer. He can’t handle it. It’s too much for him. He deserts the team and heads home at the next opportunity.
Acts 15 John Mark gets a second chance and Barnabas takes him back to Cyprus. However, he goes with the knowledge that Paul thinks it unwise to take him due to his bailing out previously. He has to prove himself and not let Barnabas down.
In fact, although Paul was the one who intially proposed a return Cyprus with Barnabas, he basically decides not to go if Barnabas insists on taking John Mark (and risking a repeat desertion), so he instead chooses Silas for a companion and heads off in a different direction to Syria and Cilicia.
In all of this, God’s hand was at work. John Mark gets the chance to try again where he failed before, Barnabas gets to follow his heart and giftings in giving him that opportunity, and they minister to the people in Cyprus. Paul and Silas form a new missionary team, encourage a second region of churches, and ultimately head further, to the unreached – through Philippi, Thessalonica, Berea, Athens, Corinth, Ephesus, and finally to Macedonia and Greece. Praise God that He can use differences and even conflict for His glory and for our good!