The only ones pushing that isolation idea in your final statement are Dispensationalists from the Pre-trib Rapture theory crowd. That idea began with John Darby in 1830s Great Britain. Apostle Paul didn't preach a different Gospel than what the other Apostles preached. And it's too bad that you aren't aware of how there are many Jews that are members of Gentile attending Christian Churches!
Not all Jewish Christians are still orthodox like the unbelieving Jews. They don't all stay apart from Gentile believers either, which Paul rebuked Peter for doing at one time. It's surprising that you're not aware of this.
Out of curiosity, when you read Acts 21:18-25, you could not tell that James distinguished between Jewish believers and gentile believers?
As usual, if you are a KJV reader, it brings out the contrast most clearly, compared to all the modern versions.
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.
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.
When you read and understand this passage literally, you mean you do not read this as James saying
- All the Jews who believe in Jesus are to keep the law of Moses, and keep it zealously.
- Gentiles who believe are to do NO SUCH THING, beyond those 4 requirements to keep the peace with the Jewish believers.
That contrast is not clear enough?
And I have not even brought in the other 3 passages that reinforced my above points about what Jewish believers must do,
James 2:24-26,
1 John 2:29,
1 John 3:7