Paul's Offense

Are you referring to the incident in Acts 18:12-17? The charge against Paul was that he persuaded people to worship God in ways that were contrary to the Law. The specifics of this charge are somewhat unclear.

It could be that the Jews believed that the claims of Christianity were contrary to the teachings of the Old Testament. Paul ran into this kind of opposition frequently (e.g., Acts 13:45-50; 14:1-6).

It is also possible that the Jews believed that Paul was encouraging people to worship in non-traditional ways, abandoning the religious traditions that had been established in prior generations. Although they used the word "law," there is no evidence that Paul taught anything contrary to the Law. In fact, he regularly affirmed the law (1 Cor. 9:8-9,20-21).

Paul taught that the Law still applied — he even called it the "law of Christ" (1 Cor. 9:21). But he also taught that it needed to be applied in new ways now that Christ had come and the Holy Spirit had been given. In doing this, he distinguished between the actual teachings of the Bible on the one hand, and the traditional interpretations and applications of the Bible on the other hand. The Jews were often offended that Paul did not require anyone, Jew or Gentile, to maintain these traditions. Still, Paul did follow the traditions himself (1 Cor. 9:20-21), and encouraged others to do so as well, for the sake of peace (Acts 16:1-3; 21:23-26; 1 Cor. 10:32-33) and the gospel (1 Cor. 9:22; 10:32-33).

There is also evidence that at least some Jews misunderstood Paul's teachings, believing that when he was refusing to make the traditions mandatory, he was rejecting the Law itself. They also appear to have believed that Paul was encouraging Jews to forsake the traditional applications of the law (Acts 21:20-21). Of course, they were wrong (1 Cor. 9:20).

So, there are a few possible answers to your question, and perhaps there are others I have not mentioned. The Bible simply does not give us enough information to know the answer with precision.