Then they said to Paul: “You see, brother, how many thousands of Jews have believed, and all of them are zealous for the law. They have been informed that you teach all the Jews who live among the Gentiles to turn away from Moses, telling them not to circumcise their children or live according to our customs.” Acts 21:20-21
Paul didn’t teach the Jews who live among the Gentiles to turn away from Moses nor would he have told them “not to circumcise their children or live according to our customs.” Later Paul would say to them: “My brothers, although I have done nothing against our people or against the customs of our ancestors, I was arrested in Jerusalem and handed over to the Romans” (Acts 28:17).
Paul did teach that keeping Jewish customs wouldn’t lead to salvation and it was for this reason that Paul saw no point in teaching Gentiles the Jewish customs. Paul taught the Gentiles what they needed to know and didn’t complicate the message by explaining things that weren’t essential.
Paul regularly kept Jewish customs when he was amongst Jews and I suspect they were meaningful to him. He had a thoroughly Jewish upbringing and now that he had become a Christian he understood the significance of these customs. So many things in the Old Testament pointed to Jesus and Paul could now appreciate the depth of meaning in these customs.
The lesson for us is to start where people are. If they have a background of Christian belief we can better explain the symbols in it, we can correct misconceptions and add meaning to what they already know. Alternatively, if they come to Christ with no knowledge of Christianity then we don’t need to complicate the message with non-essentials.