John Mark, who wrote the first Gospel (the Gospel of Mark), helped three early missionaries:
Barnabas
Paul
Peter
Barnabas and Paul took Mark as an assistant on their first missionary journey. However, Mark left them and returned to Jerusalem before the trip ended (Acts 12:25; 13:4–5, 13). Because of this, when Barnabas wanted to bring Mark on the second trip, Paul refused. This disagreement split the team: Barnabas took Mark, his cousin, while Paul chose Silas. The two pairs then went their separate ways (Acts 15:36–41).
Later, Paul and Mark reconciled, and Mark became his assistant again. In Colossians, Paul calls Mark a coworker and suggests he might soon send him to visit the church in Colossae (Colossians 4:10; see also Philemon 1:24). Even later, when Paul awaited execution in a Roman prison, he asked Timothy to bring Mark because he believed Mark would be helpful in his ministry (2 Timothy 4:11).
Mark likely helped Peter with his missionary work in Italy near the end of Peter's life. In one of Peter's letters, he affectionately calls Mark his "son" who is with him (1 Peter 5:13). Early Christian tradition describes Mark as Peter's "interpreter" and suggests that Mark got his information about the life and words of Jesus directly from Peter (Eusebius, Church History 3.39.16).
Mark's Gospel is generally considered the earliest of all the Gospels. It was almost certainly a primary source for Matthew and Luke when they wrote their own Gospels. For this reason, Mark's Gospel is one of the most influential early Christian writings.
Mark's story shows that God can fix human mistakes and mend difficult relationships for Christ and the good news. Early failures do not stop someone from having a meaningful and impactful life of service.
Passages for Further Study
Acts 12:12, 25; 13:4–5, 13; 15:36–39; Colossians 4:10; 2 Timothy 4:11; Philemon 1:23–24; 1 Peter 5:13