At first sight, the indication in Acts 13:13b that John left Paul and Barnabas to return to Jerusalem has little relevance to the developing narrative. Deliberating a little though provides us with a valuable lesson for Christian living / ministry.
In Acts 13:5b we read that John was with Paul and Barnabas as a helper. He had a valuable role to play in the drive to take forward the gospel message. They all travelling on into Asia Minor as recorded in 13:13 and then that was the time when John left them. Whey he left we are not told. Speculation and reasonable guesses abound, but the fact is he did leave. But how would Paul and Barnabas respond? They kept going. For the two of them to lose their helper in their work to take the gospel to new territory must have been a blow. But this did not stop them from pressing on.
The lesson I want to discern here bears upon how we respond when people leave the church or decide to withdraw from a ministry. What do we do in such instances? No doubt there may be consideration that has to be given to anything the person says to criticize the church or how the ministry operates, But this must not divert those still in the church or those still involved in the ministry from pressing on. There is a work to be done and a life to be lived for Christ.
When people leave, there can be a great disappointment; Paul and Barnabas were no doubt disappointed. But we must be resolved to keep going on for the Lord and not let set-backs knock us off-course.