Acts 21:20 & 21
When they heard this, they praised God. 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.'
When the leaders in Jerusalem heard about what God was doing in his ministry, they praised God but then had to inform Paul that not everyone was happy with his ministry. The thousands of Jews that believed were "zealous for the law" instead of zealous for Jesus as Paul was. Therefore, Paul was being falsely accused of turning people away from their customs. Paul was not doing that but was simply telling people that customs, rituals, or belonging to a particular group of people did not bring salvation. Sadly, we face many of the same issues today as legalism has crept back in and even taken over in some groups of believers.