Because Paul cannot yield on this point [the gospel is available to Gentiles without having to proselytize] does not mean that he opposed all things Jewish or that he would discourage Jewish Christians from following a Jewish lifestyle after they had become Christians. This stipulation that Jewish Chrsitians recognise the right of Gentile Christians to be accepted into the people of God and continue to live a Gentile (Christian) lifestyle, does not mean that such Jewish Christians as recognised this, should not also have the freedom to continue to live in a Jewish life style.It is a fascinating question. One way I try to focus it when teaching Paul is to ask my students whether they think Paul would have encouraged his fellow Christian Jews to circumcise their male children at eight days. Let's face it: over twenty or thirty years, Paul must have come into contact with many Christian Jews who were in this situation. They have just had a baby boy. Do they circumcise him? What about Paul's nephew (Acts 23.16); did Paul encourage his sister to circumcise him when he was born, and what about his brothers, or their children?
In the Campbell quotation above, it depends a bit on what we mean by the word "discourage". Given that Paul not living "under the Law" at all times, does that not act as a kind of discouragement to other Christian Jews from keeping the Law? Paul's "all things to all people" policy (1 Cor. 9.19-23) must have appeared to some as an encouragement to other Christian Jews not to keep the Law, whatever the content of Paul's actual preaching was. In other words, I find it easy to imagine how the charge of Acts 21.21 could have been made. Paul was not "teaching all the Jews who are among the Gentiles to forsake Moses, telling them not to circumcise their children nor to walk according to the customs," but the charge had a ring of truth to it if one observed Paul's actual practice.