If there’s a covenantal relationship with Israel, and a divine calling of Jew and Gentile, then Paul comes into picture who merged these two into one, with his “ingrafted branches” analogy in Rom.11.
This analogy has a scriptural base and we can find that in Is.56 among other places. Isaiah speaks about the foreigners who join Israel. It’s the word of God which states:
“Neither let the son of the stranger, that hath joined himself to the LORD, speak, saying, The LORD hath utterly separated me from his people: neither let the eunuch say, Behold, I am a dry tree. 4 For thus saith the LORD unto the eunuchs that keep my sabbaths, and choose the things that please me, and take hold of my covenant; 5 Even unto them will I give in mine house and within my walls a place and a name better than of sons and of daughters: I will give them an everlasting name, that shall not be cut off. 6 Also the sons of the stranger, that join themselves to the LORD, to serve him, and to love the name of the LORD, to be his servants, every one that keepeth the sabbath from polluting it, and taketh hold of my covenant; 7 Even them will I bring to my holy mountain, and make them joyful in my house of prayer: their burnt offerings and their sacrifices shall be accepted upon mine altar; for mine house shall be called an house of prayer for all people. 8 The Lord GOD which gathereth the outcasts of Israel saith, Yet will I gather others to him, beside those that are gathered unto him.” (Is.56:3-8)
Rashi comments on it that this strangers and eunuchs are converts. These (converted heathens) have surely a place in the house of God and in His covenant, according to this words of God. But this also implicitly states that gentiles are not the same as the people of Israel. Otherwise these converts would not be mentioned as strangers, but were fully regard as Israelites.
What we see here is that there are people who are faithful to the God and the Jews, but they are not regarded Jewish. Probably because they are not fully Torah observant. What they did do however, is keeping the Shabbat and are faithful to the covenant. And here is a very significant remark about them, they choose the things that please me. (v.4) About them is said: Blessed are they who do this. (v.2) To keep the judgement and do justice (v.1)
When the ingathering of the Israelites happens, these ones are also ingathered with them.
This must be the idea of the apostles (Acts15) that the gentiles have a place amongst the people of God. They may dwell in the tents of Jacob (Gen.9:27) and may come to Sion (Ps.87). And especially because the most profound thing ever happened since the giving of the Torah, that gentiles receive the Holy Spirit (before they convert to Judaism), the apostles believe firmly that the Messianic times were started and thought it is not good to obligate gentiles to keep the Torah to become part of the commonwealth of Israel.
In fact they created a new community within Judaism of Geriem we-Toshaviem (strangers and so-journers). This is a status of gentiles described in Leviticus 25:35. Now, since the Messianic age has began, they have spiritually the same share in Sion as the Jews, “as many as are called”. And this is what Paul called “grafted in”. Now they become part of Israel, spiritual. And in this Messianic age there’s no difference between Jew and Gentile, “no difference between Jew and Greek” (Ro.10:12) “They are one in Christ”. (Gal.3:28) It is not that the status of so-journer is of less importance. At a deeper spiritual level such a confession shows a higher devotion to God. David himself claimed this status too, in Psalm 39:13, “For I am a stranger with thee, A sojourner, as all my fathers were”. And compare what Jesus said: “But one who is the greater among you, let him become as the younger, and one who is governing, as one who serves.” (Luke22:26)
On the other side Paul warns the gentiles not to be proud of it and not to think they are now the replaced Israel, because that was certainly not the case. It even doesn’t abolish or change the existing covenant with the Jews.
Now if we see Jews and Gentiles, separate on one plane, together on another plane, we can understand the message Paul gives us much better.