Acts 21

Take them and be purified with them, and pay their expenses so that they may shave their heads, and that all may know that those things of which they were informed concerning you are nothing, but that you yourself also walk orderly and keep the law. v.24

Read chapter 21
Paul and many believing Jews are still keeping the law.  The Law wasn't done away with just because Jesus came.  Remember He came to fulfill it.  The whole point of Paul preaching to the Jews among the Greeks was that the law was no longer a burden but of freedom in relational and passion.  That to do what they could continue to do but not feeling guilty of what they're not able to do.  Since Paul was able to keep the law way more than other Jews because of living near the temple and growing up in that way these belong Jews want to make sure that he doesn't cause men to justify by what he does/doesn't do but what the Lord has abled him to.  
What areas of your life do you justify by comparing with other believers rather than what God's put on your heart?