Acts 18:18–23 (ESV)
After this, Paul stayed many days longer and then took leave of the brothers and set sail for Syria, and with him Priscilla and Aquila. At Cenchreae he had cut his hair, for he was under a vow. And they came to Ephesus, and he left them there, but he himself went into the synagogue and reasoned with the Jews. When they asked him to stay for a longer period, he declined. But on taking leave of them he said, “I will return to you if God wills,” and he set sail from Ephesus.
When he had landed at Caesarea, he went up and greeted the church, and then went down to Antioch. After spending some time there, he departed and went from one place to the next through the region of Galatia and Phrygia, strengthening all the disciples.
The vow Paul was under appears to be a Nazirite vow.
When either a man or a woman makes a special vow, the vow of a Nazirite, … All the days of his vow of separation, no razor shall touch his head. Until the time is completed for which he separates himself to the LORD, he shall be holy. He shall let the locks of hair of his head grow long. … And this is the law for the Nazirite, when the time of his separation has been completed: he shall be brought to the entrance of the tent of meeting, and he shall bring his gift to the LORD — Numbers 6:2b, 5, 13–14a (ESV)
To complete his vow, Paul needs to offer a sacrifice at the temple in Jerusalem. You can almost feel this hanging over him in the verses between now and when he gets there.
Now after these events Paul resolved in the Spirit to pass through Macedonia and Achaia and go to Jerusalem, saying, “After I have been there, I must also see Rome.” — Acts 19:21 (ESV)
It’s interesting that Luke treats this vow almost as a secret. The only mention is, “At Cenchreae he had cut his hair, for he was under a vow.” That makes sense, in light of Matthew 6:5-6
And when you pray, you must not be like the hypocrites. For they love to stand and pray in the synagogues and at the street corners, that they may be seen by others. Truly, I say to you, they have received their reward. But when you pray, go into your room and shut the door and pray to your Father who is in secret. And your Father who sees in secret will reward you.
Vows aren’t for show.
It is good to praise someone when they’ve done something well and it feels good to be praised—but there’s a fine line there that we need to be aware of. The praise is healthy, but our reaction sometimes isn’t.
We mustn’t crave that praise, and we mustn’t resent it when it’s lacking. Jesus tells us how to battle this emotion—hide your works.
Keep your works and gifts to the church and other charities secret. For example, when you say grace in a restaurant, be discrete.
