New Defender's Study Bible Notes
18:18 shorn his head. For some reason, Paul had apparently taken a Nazarite vow, not cutting his hair until the duration of the vow was finished. Possibly it was a vow of thanks for God’s promise of protection (Acts 18:10). Although this was strictly a Jewish institution (Numbers 6), Paul often made an effort to retain his Jewish identity, hoping thereby to reach the Jews more effectively (note I Corinthians 9:19-20; Acts 16:3; 21:18-26).