18:18 he was under a vow. Though this phrase could apply to Aquila, Paul is probably in view. The Nazirite vow required rigorous ceremonial purity that would have been impractical in Gentile lands (Num. 6:1–21), so this vow was more likely a private one undertaken by Paul as a religious exercise. The hair was allowed to grow during the period of the vow, and cutting it marked the conclusion of the vow and was perhaps an expression of gratitude to God.