So now that you have looked at Acts 21 let's break some portions down.
Act 21:17 When we had come to Jerusalem, the brothers received us gladly.
Act 21:18 On the following day Paul went in with us to James, and all the elders were present.
Act 21:19 After greeting them, he related one by one the things that God had done among the Gentiles through his ministry.
Act 21:20 And when they heard it, they glorified God. And they said to him, "You see, brother, how many thousands there are among the Jews of those who have believed. They are all zealous for the law,
Act 21:21 and they have been told about you that you teach all the Jews who are among the Gentiles to forsake Moses, telling them not to circumcise their children or walk according to our customs.
Act 21:22 What then is to be done? They will certainly hear that you have come.
When Paul arrives in Jerusalem and meets with James and the elders they received him gladly. And when he gave them the details of his ministry with the Gentiles the glorified God. But they immediately had reservations because of the rumors they had heard. They point out that there are many thousands of Jews who are believers in Yeshua and "they are all zealous for the law". Hint to Paul, believing in Yeshua has not altered the Jews from following the Torah. And tightly so, Yeshua was a Torah observant Rabbi. So, do you think they are upset for Paul teaching the Gentiles to observe the Torah? No, the Gentiles are in their synagogues learning Torah.
But let's look at a few more verses.
Act 21:23 Do therefore what we tell you. We have four men who are under a vow;
Act 21:24 take these men and purify yourself along with them and pay their expenses, so that they may shave their heads. Thus all will know that there is nothing in what they have been told about you, but that you yourself also live in observance of the law.
Act 21:25 But as for the Gentiles who have believed, we have sent a letter with our judgment that they should abstain from what has been sacrificed to idols, and from blood, and from what has been strangled, and from sexual immorality."
James and the elders had already judged that the Gentiles did not have to be circumcised to be accepted into the faith of Yeshua or accepted into the synagogues. But the Jewish believers were under the impression that Paul was telling the Jewish believers that they should not be circumcised or observe Jewish customs.
Let's take a moment and look at the custom of the Temple. The Gentiles were allowed in the Temple's outer courts. In fact, the largest of all the courts was the great court of the Gentiles. But they could not enter into the inner courts of the Temple. There was a great dividing wall that stood between them. Josephus describes, "There was a partition made of stone all around, whose height was three cubits; its construction was very elegant; upon it stood pillars, at equal distances from one another, declaring the laws of purity, some in Greek, and some in Roman letters, that 'no foreigner should go within that sanctuary under pain of death."
Think of this, there were thousands of believing Jews but none of them were being dragged before the Sanhedrin. There is a message from Paul more controversial than the resurrection of Yeshua.
If you want to get ahead, notes and discussion are being presented from; Grafted In Israel, Gentiles and the Mystery of the Gospel, D. Thomas Lancaster.