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;
All Commentaries on 1 Corinthians 9:20 Go To 1 Corinthians 9
John Chrysostom
AD 407
Paul did not become a Jew in reality but only in appearance. How could it have been otherwise, since he was so determined to convert them and deliver them from their predicament? Homilies on the Epistles of Paul to the Corinthians