When [Paul and Barnabas] arrived at Salamis, they proclaimed the word of God in the synagogues of the Jews. And they had John to assist them.
Acts 13:5
NIV
Now Paul and his company set sail from Paphos, and came to Perga in Pamphylia. And John LEFT THEM and returned to Jerusalem.
Acts 13:13
NIV
Barnabas wanted to take John, also called Mark with them [on their next trip]. But Paul did not think it wise to take him because he had deserted them in Pamphylia, and had not continued with them in their work. They had SUCH A SHARP DISAGREEMENT that they parted company; Barnabas took John Mark with him and sailed for Cyprus, but Paul chose Silas and left.
Acts 15:37-40
NIV
Walk with the wise and become wise; associate with fools and get in trouble.”