Acts 21:23-24

23Do therefore what we tell you. We have four men awho are under a vow; 24take these men and bpurify yourself along with them and pay their expenses, cso that they may shave their heads. Thus all will know that there is nothing in what they have been told about you, but that you yourself also live in observance of the law.

1 Corinthians 9:20

20 dTo the Jews I became as a Jew, in order to win Jews. To those under the law I became as one under the law (though not being myself under the law) that I might win those under the law.
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