Years after the Jerusalem council, Paul again traveled to Jerusalem where he testified before James and the elders of all things that GOD had done among the gentiles.
And when they heard this, they began to glorify God. And they said to [Paul], “You see, brother, how many ten thousands there are among the Jews who have believed, and they are all zealous adherents of the law. Acts 21:20
As touching the Gentiles which believe, we have written and decided that they observe no such thing, save only that they keep themselves from things offered to idols, and from blood, and from strangled, and from fornication. Acts 21:25 Acts 21:25
Well, you left out quite a bit of Scripture that shows what was going on there.
Acts 21:18-25
18 And the day following Paul went in with us unto James; and all the elders were present.
19 And when he had saluted them, he declared particularly what things God had wrought among the Gentiles by his ministry.
This is Apostle James speaking to Paul:
20 And when they heard it, they glorified the Lord, and said unto him, "Thou seest, brother, how many thousands of Jews there are which believe; and they are all zealous of the law:
21 And they are informed of thee, that thou teachest all the Jews which are among the Gentiles to forsake Moses, saying that they ought not to circumcise their children, neither to walk after the customs.
22 What is it therefore? the multitude must needs come together: for they will hear that thou art come.
23 Do therefore this that we say to thee: We have four men which have a vow on them;
24 Them take, and purify thyself with them, and be at charges with them, that they may shave their heads: and all may know that those things, whereof they were informed concerning thee, are nothing; but that thou thyself also walkest orderly, and keepest the law.
25 As touching the Gentiles which believe, we have written and concluded that they observe no such thing, save only that they keep themselves from things offered to idols, and from blood, and from strangled, and from fornication.
KJV
That was James speaking all that to Paul, including that 25th verse. That shows nothing changed from what the Apostles at Jerusalem had commanded the Gentiles to keep per Acts 15.
If you look at Acts 21:21 there more closely, you'll see why James and those Jewish elders (that were believers) were in such a stir from Paul's teaching. They claimed Paul had been teaching believing JEWS AMONG THE GENTILES to not heed God's laws anymore.
Do I believe Apostle Paul was really... intending to do that upon the Jewish believers among the Gentile believers? No. Paul did not teach against keeping God's laws anywhere in his Epistles.
Paul taught that none shall be justified or saved by keeping the law, which is different than saying not to try and keep them at all. The believing Jews among the Gentiles evidently thought Paul was telling them to not... keep the law, period.
That's apparently how the Jewish believers among the Gentiles misunderstood Paul's teaching. The dispute that began in Acts 15 was different, because 'some of the Jewish believers' (not Paul), were teaching Gentiles had be circumcised to be saved in Christ. And the conclusion of the Apostles at Jerusalem for what the Gentiles were to follow was the same as this verse 25 here.
Then James suggested that Paul along with those other four Jews perform that purification rite per the law, as an act of fellowship with the Jewish Church at Jerusalem. And that Paul did, which is perfectly fine, since Apostle Paul himself was a Jew born of the tribe of Benjamin.
For one born of Jewish heritage, even as a believer on Christ Jesus, it is perfectly fine for them to keep God's laws. They are NOT... to seek to be justified in Christ by doing that though, because Christ's Salvation is NOT by following law, but instead a free gift by His Grace for those who believe. Nor are they to teach Gentiles that they must keep God's laws to be saved in Christ Jesus. It's that simple.
The reason why it is still OK for believing Jews to keep God's laws, is because God promised a lawgiver would not depart from Judah all the way up to the time of Christ's 2nd coming (Gen.49:10; "Shiloh" represents our Lord Jesus there). Thus our Heavenly Father has ordained His laws to remain in their hands and responsibility until Jesus returns.
Thus, in my posts on this matter, I have had no... arguments with those above principles.
Instead, my purpose (even as a Gentile believer on Christ Jesus), has been to help educate regarding why God's laws (portion that Christ did not nail to His cross) are still in effect today, as many of them are established in the societies of the western nations, and that since those western nations as a majority believed on Jesus Christ very quickly after His death and resurrection, and He was preached to them (Britain being the first on a national scale to accept The Gospel and put away their old paganism). As a result, the early history of the western Christian nations implemented many things directly from God's laws in the OT to govern Christian society. They, like Apostle Paul, well understood how portions of God's laws are for controlling the wicked and lawless, like Paul showed in 1 Timothy 1.
Brethren would do well to research the history of Blue Laws that used to be established in the early American colonies and early history of the United States. In many parts of the U.S., non-essential businesses were required to close on Sundays, which was considered as a Christian sabbath, an idea they got directly from God's laws in the OT. Those were Christians in the majority who ordained that for themselves and their communities. And of course the lawless and wicked really hated all that.