Acts 21:17 - 19

"When we arrived at Jerusalem, the brothers received us warmly. The next day Paul and the rest of us went to see James, and all the elders were present. Paul greeted them and reported in detail what God had done among the Gentiles through his ministry." NIV translation

Paul made a courtesy call on the leaders of the church in Jerusalem. I say this because there are many who take his visit here and try to make a case for a certain "church" structure and even to go so far as calling a man on earth the "head of the church". 

In Ephesians 1, Paul explained that God's will is for Jesus to be the head of the church and not a man here on earth. Paul worked with the leaders of the believers in Jerusalem but he was subject to Jesus Christ and listened to the Holy Spirit for direction. He simply informed them about what God had been doing in his life. (We would call this sharing a testimony.)

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.'" NIV translation

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.