The ceremonial law died with Christ, Ephesians 2:15,16, wherefore Christians were not obliged to the performance and observation of it after the death of Christ; but it pleased God for a time to indulge the Jews in the observance of those rites, until they could clearly see, and be fully persuaded of, their liberty from it, with which Christ had made them free; and it was some good time before all those, who from Judaism had turned to Christianity, could be thus persuaded, as we may learn from Galatians 4:21, they desired to be under the law. To such, saith the apostle, I became as a Jew, that is, I observed some rites which the ceremonial law (peculiar to the Jews) required; an instance of which we have, Acts 21:23-26, where we find Paul purifying himself (according to the rites of the ceremonial law) with four men which had a vow upon them. The Jews before Christ's death were under the law; many of them, though converted to the Christian religion after the death of Christ, apprehended themselves under the law, not as yet seeing the liberty with which Christ had made them free: saith the apostle, I, knowing the will of God, for a time, that the Jews should be indulged as to their weakness, became as one of them under the law, that I might gain them, that is, reconcile them to the Christian religion, and in some measure prepare them for the receiving the gospel. We have an instance of this in Paul's practice, Acts 16:3, where he circumcised Timothy, because his mother was a Jewess, that he might not irritate the Jews in those quarters, nor estrange them from the doctrine of the gospel. In all this Paul did nothing that was sinful, but only determined himself as to the liberty which God had given him, when he might do or forbear, either doing or forbearing to do, as he saw the one or the other made most for the honour and glory of God in the winning of souls.
Related Commentaries of 1 Corinthians 9:20
1 Corinthians 9:20
20 And unto the Jews I became as a Jew, that I might gain the Jews; to them that are under the law, as under the law, that I might gain them that are under the law;