Jesus is the mediator between God and man.
For there is one God and one mediator between God and mankind, the man Christ Jesus,
That's true but the Covenant God made was with Abraham the person. God made no covenant with any Gentile person.
The circumcision of the new covenant is inward not outward on the heart of the believer by the Spirit of Christ. And has no boundaries that is male and female Jew and gentile.
Abraham received the covenant with God both naturally and spiritually.
Gentiles enjoy the blessings of the Abrahamic Covenant but not the covenant naturally. Abraham and his descendants (Isaac, Jacob, the twelve sons and their descendants) have covenant with God in which the covenant is to the people that would become the children of Israel. The covenant culminated in the ONE Seed that would come (Christ) in which all the Promises would be fulfilled.
Jesus is Israel's Messiahs' human name. Christ, or Messiah, is His title and it means "anointed." Thus, through Christ the spiritual blessings to Abraham are fulfilled so that God can save Israel through the Promised Holy Spirit.
God's covenant with Abraham is to a person but God has made no similar or any otherwise covenant with any Gentile people. The whole covenant begins with Israel, is interrupted by Gentiles, and concludes with Israel.
Those who are born of God are those who God calls His children.
God calls Israel His son. Gentiles are adopted.
Yet to all who did receive him, to those who believed in his name, he gave the right to become children of God.
Those that reject the Son don't have the Father either.
No one who denies the Son has the Father; whoever acknowledges the Son has the Father also..
When all is said and done then that statement will be realized. Until then, God is bringing in Gentiles to Himself through Christ who is the fulfillment of both the natural and spiritual aspects of the Abraham Covenant. As King He will have the land and the subjects and all power and authority and will rule this world and the universe from Jerusalem, Israel.
So Gentiles have Sonship, the same inward circumcision of the Spirit on their hearts and Jesus is their mediator and the same promises/blessings from God received in like manner by Faith in the Son through Gods grace. If you don't want to call that the same covenant relationship as those who believers were outwardly circumcised from Israel that's fine with me but I disagree. Rejection of the Son also brings the same curses for all. The wrath of God remains on them.
The same blessings and curses for all.
Show me a covenant in Scripture is which God says the following thus establishing a covenant:
7 And
I will establish my covenant between me and thee and thy seed after thee in their generations for an everlasting covenant, to be a God unto thee, and to thy seed after thee. Gen. 17:7.
There is none. Gentiles are grafted in 'after the fact' and receive the BLESSINGS of Abraham but not the covenant per se as God has made no covenant with Gentiles.
Yet not tied to the theocracy of Israel which was disbanded as Jesus prophesied the consequences for rejecting Him that is their house is left desolate.
Before Jesus ascended his disciples asked Him:
6 When they therefore were come together, they asked of him, saying, Lord, wilt thou at this time restore again the kingdom to Israel Acts 1:5–6.
For 40 days before Jesus ascended, He did this:
3 To whom also he shewed himself alive after his passion by many infallible proofs, being seen of them forty days, and
speaking of the things pertaining to the kingdom of God: Acts 1:3.
Jesus was Israel's long-awaited promised Redeemer and King. Through Him He would eventually united both the northern and southern kingdoms in Himself thus making two into one. He will gather Gentiles and graft them into the natural branches and make these two into one.
Everyone in Israel right up to the time of Jesus' arrest received Him as Israel's Messiah and King. They wanted to take Him by force and MAKE Him King. That's the only thing on their minds and on their lips as the disciples' question of restoring the kingdom to Israel reveals.
Their "house" (Temple) is left desolate because Christ is not in it, not because Israel rejected Him. But whether Jews or Gentile believers commit whoredom and practice idol-worship and witchcraft and break their vows of chastity and faithfulness, God is faithful and will keep ALL promises made to Israel.
One day the Times of the Gentiles will end, and God will turn His full attention back to Israel and bring her back to Himself and He will do this through the preaching of the two witnesses of Revelation and the Holy Spirit and their Law, Psalms, and Prophets: The Old Testament. And then all Israel shall be saved.
Israel is God's Bride.
Israel is God's Church.
At the Marriage Supper Israel will be there through covenant.
Gentiles will be there through invitation.