Final answer:
John Mark left the group due to a disagreement between Paul and Barnabas during their missionary journey.
Step-by-step explanation:
According to the New Testament, John Mark left the group because of a disagreement between Paul and Barnabas. At the time, Paul and Barnabas were on a missionary journey together, and John Mark was with them as an assistant. However, John Mark decided to leave and return to Jerusalem, which caused a rift between Paul and Barnabas, resulting in them parting ways.
One possible reason for John Mark's departure could be the challenging nature of the journey or the pressure of the missionary work. Another reason could be personal differences or conflicting visions for the mission. However, it's important to note that the exact details of why John Mark left are not explicitly mentioned in the Bible.