So salvation is all by grace. That is the conclusion reached by the council of Jerusalem. And new Gentile believers do not have to become Jews, put themselves under the Law of Moses or be circumcised (phew) to be proper believers in God. God has received them by faith and given them the Holy Spirit. No works required.
Or are they...?
We get to the odd little verses 15:19-20.
First, a pastoral conclusion from James: let's not make it difficult for Gentiles who are turning to God. But then, let's insists that they abstain from food polluted by idols, from sexual immorality, from the meat of strangled animals and from blood. Oh, so there are works you have to do, then? AND it seems to be based not on the gospel but on Moses, who has been preached from the earliest times (v21).
What are we to do with these instructions on behaviour for Gentile believers, based on Moses? Are they a new Law? A summary of the Mosaic Law showing that they actually do have to obey?
Thankfully, no. The New Testament is very clear that Christians are not under the Law for justification and not under the Old Testament food regulations as part of the Law. Check it out in Acts 10, where God declares all food clean. Christians may eat anything. But are there circumstances under which we it would be better not to? Yes, and the verse that explains it best is Romans 14:20-21:
All food is clean, but it is wrong for a man to eat anything that causes someone else to stumble. It is better not to eat meat or drink wine or to do anything else that will cause your brother to fall.
That's it. Brotherly kindness and respect to the consciences of newly Christian Jewish believers is the reason for the stipulations from the Jerusalem counsel. "Do this because Moses has been preached, people are sensitive about these things and have a conscience about them. Don't exercise your freedom in a blatant way at the expense of their conscience."
If the Jerusalem Jewish believers are told to make it easy for the new Gentile believers, and do so in love and at high personal cost, then this is the call to the new Gentile believers to make it as easy as they can on the consciences of the Jewish believers.
Whenever change is afoot there will be tender consciences and people who resist change simply because any change challenges received ideas of godliness. That's good: innovation should be tested and ought to stand up to the scrutiny of people who want to be godly (even if they have always understood that through a certain cultural lens). During the process of transition it is vital for the "Jerusalem church" to make it as easy as possible for the "new Gentile believers" and for those believers in turn to be as sensitive as possible to tender consciences.