Acts 18:18-21 :
Paul, having stayed after this many more days, took his leave of the brothers, and sailed from there for Syria, together with Priscilla and Aquila. He shaved his head in Cenchreae, for he had a vow. He came to Ephesus, and he left them there; but he himself entered into the synagogue, and reasoned with the Jews. When they asked him to stay with them a longer time, he declined; but taking his leave of them, and saying, “I must by all means keep this coming feast in Jerusalem, but I will return again to you if God wills,” he set sail from Ephesus.
Christianity is not a legal system. When Paul decides to take an oath or keep a feast, we shouldn’t think that he is doing it to follow the Law of Moses. Paul knew very clearly that this was not necessary. Don’t be swayed into thinking that because Paul decided to follow self-imposed rules that he was implying that we should keep the Law. That would go against large amounts of what Paul clearly wrote in his letters that are now in our Bible.
On the other hand, Christianity is a free system. When Paul decides to take an oath or keep a feast, we shouldn’t think that He is wrong because he is keeping a part of the Law of Moses. Paul, being a Christian, is perfectly free to take an oath or keep a feast if he chooses to. Whatever He does is between him and God. We would be hypocrites to say that Paul shouldn’t keep a feast or an oath because it is a law, because to do so would be to enforce a law on Paul!
Sometimes, we have personal reasons for doing certain things and what we do is between ourselves and the Lord as long as those things are not against the Law of God. Notice what Paul said about the fruit of the Spirit in Galatians:
Galatians 5:22-23 :
But the fruit of the Spirit is love, joy, peace, patience, kindness, goodness, faith, gentleness, and self-control. Against such things there is no law.
Paul’s focus wasn’t on what He couldn’t do but what He could. He could choose to do anything out of love that He wanted to do. Perhaps taking an oath was a personal discipline that the Holy Spirit was involved in helping Paul to keep. Perhaps going to the feast meant that he would be able to witness to old friends. Whatever the case may have been, we were not told and it really isn’t our business. Paul was clearly not doing anything that violated God’s law.