When they were about to begin their second missionary journey Paul and Barnabas had a falling out over John Mark, Barnabas' nephew. Though his name is mentioned in several of Paul's epistles, we read nothing else of significance about him in the New Testament, except that he was carried away with the dissimulation of Peter and other Jews at Antioch (Gal. 2:1, 9, 13). Where he went, what he did, what became of him after this, we are not told.
This much is certain: As far as effectiveness and usefulness is concerned Barnabas' ministry ended when he stubbornly refused to submit his love for family to the cause of Christ, when he chose to defend Mark in his error, even if it meant sacrificing the work to which God had called him (Acts 13:2). What a sad end for a man who had such a brilliant beginning and had been used for so much good! Barnabas sacrificed everything he had once counted dear because of family ties. He who once sold all he had and gave the money to the church (Acts 4:36-37), in the end gave up his place of service in the cause of Christ to defend his kinsman!
The lesson is clear. As far as God's servant is concerned, his family is and must be the family of God. All earthly relations and worldly concerns must be subservient to the church of God, the gospel of God, and the glory of God. No man can serve two masters. We cannot serve our own interests and Christ's.