It is interesting that Jesus is the first-born and we are the first-born with Him (Rom 8:29), but we are not the first-born of Jesus; we are the first-born of God. God is our Father (I Cor 1:3, II Cor 1:2, Eph 1:2), not Jesus. We are married to Jesus (Eph 5:32), but are not His children. The Spirit that He gave to us and by which we are begotten came from His Father (I Jn 5:18, Jas 1:17-18, I Cor 12:8, II Pet 1:21). Like Joseph had many brothers, so does Jesus (Rom 8:29). Like Joseph was the firstborn among his brethren, even so Christ is the firstborn among us. He gets a double portion in His kingdom and the blessing of the Father. Our first-born status is not the rebirth we experience at baptism, but is the rebirth we experience at the resurrection from the dead (Acts 13:33-34, Rev 1:5, Col 1:18). When God and Christ come again, the dead in Christ rise first (I Th 4:16). We are therefore the firstborn ones on the new earth . After the resurrection and Judgment, God’s kingdom expands without bounds for ever (Isa 9:7), and those that come after us will regard us as we do Adam. He is the father of all living now. The firstborn ones (Heb 12:23) will be the fathers of all living in the world to come. Christ is the second Adam from whom we in the church are taken from His side and become the new Eve, His wife. Together in the world to come Christ and the church will be the new Adam and Eve.
It is the established position among the brethren that the Christians are the new Jews. It is true that we will inherit the promises God promised the Seed (Gen 13:15), but we do so because we are married to the true Heir, Jesus, the Messiah, not because we are the new Jews. The position that Christians are the new Jews is called replacement theology. I don’t see evidence for it. I find that “Jews” and “Israel” is almost always used literally in the NT. In Ro 9:6 the term “Israel” is used to stand for “the elect”. In that sense, the elect have always been a smaller portion of Abraham and Isaac’s descendants, as Paul demonstrates from the historical types of Isaac and Ishmael (Rom 9:7), Jacob and Esau (Rom 9:13), and also from the prophet Isaiah (Rom 9:27). The true “Jew” has always been the one that elected to serve God. It was never enough to be a descendant of Abraham (Mt 3:9). The descendant of Abraham had to choose to obey God’s will after he came of age. Then and only then could this one that was born of Israel become a part of true Israel by a personal choice to obey God’s law. The patriarchs (Adam, Enoch, Noah, etc.) became part of this number by their choice to serve God. Those that become Christians become part of this number of true Israel by likewise choosing to serve God. So also do the Jews.
What changed when Christ came was that God took the kingdom away from Israel (Mt 21:43). I understand that kingdom to be comprised of Christ the King, Christ’s law, God’s throne, the territory in heaven, and the church as well as the spirit beings who are in obedience to God that are Christ’s subjects. Our King and kingdom is currently in heaven (Acts 2:34-35, Php 3:20). We are strangers, sojourners and pilgrims in the present age because the present earth is in rebellion to God. Heaven is not, and that is where we go when we die (Php 1:23, II Cor 5:8-9). When God comes again, He brings the spirits of the righteous dead (I Th 4:14) whom He then raises as the firstborn on the new earth.
However, the fact that God took the kingdom from Israel does nothing to negate the promises God made to Israel that they would inherit the land, have a great name, be numerous, and be a blessing. All of these things are still true and will also be true in the new earth, for the promises of Abraham are “an everlasting covenant” (Gen 17:7). There are other promises to Israel and prophecies of Israel that cannot be fulfilled if God has completely given them up. God also specifically said that anyone that touched Israel touched the pupil of God’s eye (Zech 2:8), and that prophecy is in the context of the new earth.