Firstly, i want to quote paul in Acts 26:4-8 where he is standing before Herod Agrippa and in his own words he gives us his mission statement
“My manner of life then from my youth up, which was from the beginning among mine own nation and at Jerusalem, know all the Jews;
5having knowledge of me from the first, if they be willing to testify, that after the straitest sect of our religion I lived a Pharisee.
6And now I stand here to be judged for the hope of the promise made of God unto our fathers;
7unto which promise our twelve tribes, earnestly serving God night and day, hope to attain. And concerning this hope I am accused by the Jews, O king!
8Why is it judged incredible with you, if God doth raise the dead?”
i quote this to give proper context that every nation that Christ instructed him to bring the gospel to were in fact of the seed of abraham, they were the Israelites whom Yahweh divorced and were of the house of Israel. The epistle to the Galatians was written to the Celtic peoples who migrated from Gall of whom were Israelites of the dispersion of which Paul was aware to this fact as well as Peter where in his epistle he opens the letter writing
“
1Peter, an apostle of Jesus Christ, to the elect who are
sojourners of the Dispersion in Pontus,
Galatia, Cappadocia, Asia, and Bithynia,
2according to the foreknowledge of God the Father, in sanctification of the Spirit, unto obedience and sprinkling of the blood of Jesus Christ: Grace to you and peace be multiplied. 1 Peter 1:1,2
In response to the scripture that you quoted Paul is using the phrase “jew nor Greek” which most believers assume means “of the 12 tribes nor everybody else” and this is false because at the time Paul is teaching there were only the tribes of Benjamin,Levi and Judah of the circumcusion in Judea which the term “Jew” is referring to and where Paul chooses the term “Greek” these were of northern 10 tribes which had been scattered and unto whom Paul was sent to bring the Gospel to which was prophesied by Jeremiah
saying in Jeremiah 31:10
““Hear the word of the LORD, you
nations; proclaim it in distant coastlands: ‘He who
scattered Israel will gather them and will watch over his flock like a shepherd.’”
and in the same chapter of Jeremiah we read about this new covenant..
“Behold, the days come, saith Jehovah, that I will make a new covenant with the
house of Israel, and with the house of Judah:
32not according to the covenant that I made with their fathers in the day that I took them by the hand to bring them out of the land of Egypt; which my covenant they brake, although I was a husband unto them, saith Jehovah”. Jeremiah 31:31,32
in the first quote i highlighted “nations” because this is the tragically mistranslated word “gentiles” which should always be translated as “nations”. These nations are the nations that were promised unto the forefather Abraham of which the northern tribes the house of israel were scattered into with the assyrian captivities but in Yahweh Gods divine providence those scattered Israelites became the promised nations of Abraham. I also highlighted “the house of israel and the house of judah” because the new covenant was only for them and nowhere in scripture is it stated otherwise. Paul is telling those Jews who were circumcised that it no longer mattered as to their Greek brethren, the Israelites of the dispersion of whom were scattered and given up to their sin, divorced of Yahweh driven out into paganism but now they were being called back to their God and redeemed by the precious blood of their kinsman redeemer Jesus Christ. If you notice the sheperd and flock terminology used by the prophets was intentional as Jesus being the Good Sheperd who came for His sheep.
I didnt use more scripture because their is so much spoken just by the prophets alone but as you study more intensively you will begin to pick up on all the old testament scriptyre that paul quotes all throughout his epistles and this done obviously to prove the fulfillment of the work of christ. There is a whole other task of study that i would stress upon every one of my brethren and that is intensive historical research regarding Israel and the surrounding nations from the time which they came out of Egypt, through the assyrian and babylonian captivities into Christ ministry.
If you have further questions I would love to discuss anything and everything about the scripture. I really hope that this opens your mind and your heart. Praise Yahweh