I will come back if it is God’s will

Acts 18:18-28
When they asked him to spend more time with them, he declined. But as he left, he promised, “I will come back if it is God’s will.”
Apollos, he began to speak boldly in the synagogue. When Priscilla and Aquila heard him, they invited him to their home and explained to him the way of God more adequately.
When Apollos wanted to go to Achaia, the brothers and sisters encouraged him and wrote to the disciples there to welcome him. When he arrived, he was a great help to those who by grace had believed. For he vigorously refuted his Jewish opponents in public debate, proving from the Scriptures that Jesus was the Messiah.
A lot of travelling is packed into this short passage. Paul goes by sea from Corinth via Ephesus to Jerusalem and Antioch. He then returns to Ephesus by road through Phrygia and Galatia for a further missionary campaign.
Cenchreae was the port for sailing east from Corinth. 'Jews made vows to God either in thankfulness for past blessing or as part of a petition for future blessings. Under a Nazirite vow, hair was cut only when the vow came to and end and a sacrifice was offered.
Why?
Do not promise to the future, actually I will ... if it is God's will. Humble before the Lord.
If someone does not clear to the Jesus Christ we should explained to him the way of God more adequately like Priscilla and Aquila did to Apollos. Not only teaching but also encourage him.
So it may build a good servant of the Lord to help to those who by grace had believed.
How?
Be humble, for I am under the authority, my future is under God's promise.