Adam Clarke on Acts 21:21

Thou teachest - to forsake Moses, etc. - From any thing that appears in the course of this book to the contrary, this information was incorrect: we do not find Paul preaching thus to the Jews. It is true that, in his epistles, some of which had been written before this time, he showed that circumcision and uncircumcision were equally unavailable for the salvation, of the soul, and that by the deeds of the law no man could be justified; but he had not yet said to any Jew, forsake Moses, and do not circumcise your children. He told them that Jesus Christ had delivered them from the yoke of the law; but they had, as yet, liberty to wear that yoke, if they pleased. He had shown them that their ceremonies were useless but not destructive; that they were only dangerous when they depended on them for salvation. This is the sum of what Paul had taught on this subject.