Paul left the version of Jewish religious practice he was a part of.
This is one interesting thing about that, people tend to confuse their form of religious pratices with being properly in God's will. There were several forms of Jewish practices back in the day. Denominations if you will.
So, some Jews thought Paul was an apostate. Some didn't. God also didn't. That is the important part, God didn't think Paul was an apostate, and even more, the Law of Moses talked about Christ, Paul being one of the few who knew it.