Acts 18:18

A vow] After delivery from danger or recovery from sickness, the Jews were accustomed to take upon themselves a modified form of the Nazirite vow (see Numbers 6). As the special consecration of this state forbade intercourse with Gentiles, St. Paul deferred it till his work at Corinth was finished. The essential ceremony was the presentation of the hair grown during the period of separation at the altar at Jerusalem together with certain specified sacrifices; hence the head was shaved both at the beginning and at the end of the period of separation. See further Acts 21:26. It is not necessary to suppose that St. Paul took this vow to conciliate the Jews or the Jewish Christians. He simply adopted the usual Jewish way of thanking God for a great deliverance.

Many additional particulars about the Corinthian ministry of St. Paul can be learnt from 1 and 2 Cor. See the commentary on those Epistles.