WHO IS GOD'S ISRAEL?
Jesus Christ, who is God, but who came as man, came via the line of Judah, son of the patriarch, Jacob, renamed Israel.
1I say the truth in Christ, I lie not, my conscience also bearing me witness in the Holy Ghost, 2That I have great heaviness and continual sorrow in my heart. 3For I could wish that myself were accursed from Christ for my brethren, my kinsmen according to the flesh: 4Who are Israelites; to whom pertaineth the adoption, and the glory, and the covenants, and the giving of the law, and the service of God, and the promises; 5Whose are the fathers, and of whom as concerning the flesh Christ came, who is over all, God blessed for ever. Amen. 6Not as though the word of God hath taken none effect. For they are not all Israel, which are of Israel:
Two famous sons of Abraham Ishmael and Isaac: Just as everyone of the promise does not include all descendants of Abraham, it takes more than to be a descendant of Israel or Judah to be of God's Israel. God reveals to us that God is not bound to the culture of man or by man's fleshy rules.
7Neither, because they are the seed of Abraham, are they all children: but, In Isaac shall thy seed be called.
All bloodline descendants of Abraham or Israel are not the children of the promise. All bloodline descendants of Judah are not the children of the promise. The purpose of Roman 9:7 is not to exclude all descendants of Ishmael, but to show that with God, blood or flesh is not the criteria for being God's People. Love and trust for YHWH has always been the criteria for children of the promise. Originally, while physical Israel was the seed of Abraham, the Israel of God was based on love and trust in YHWH and faith in God’s promise, not bloodline alone.
8That is, They which are the children of the flesh, these are not the children of God: but the children of the promise are counted for the seed.
Throughout the Bible, we are told that God chooses to have mercy on ALL of the human race, for God loves ALL. Abraham is the father of the faithful. The faithful are the children of the promise. Paul is not changing the words of YHWH or Jesus. All, including heathens, become the children of God through faith when they trust God and have faith in the works of Jesus Christ. Only the faithful remnant from the OT (Abraham and the prophets) looked forward to Jesus's Day.
30What shall we say then? That the Gentiles, which followed not after righteousness, have attained to righteousness, even the righteousness which is of faith. 31But Israel, which followed after the law of righteousness, hath not attained to the law of righteousness. 32Wherefore? Because they sought it not by faith, but as it were by the works of the law. For they stumbled at that stumblingstone; 33As it is written, Behold, I lay in Sion a stumblingstone and rock of offence: and whosoever believeth on him shall not be ashamed.
Today, the term “Jew” refers to all members of the Jewish religion. However, the Bible clearly speaks of the bloodline descendants of Judah, not just religious Jews. Thus, the translation “Jew” and "Jewish" creates confusion. Nevertheless, God has mercy on all who will have faith in his promise—in his chosen Savior of the World—Jesus the Christ.
26For ye are all the children of God by faith in Christ Jesus. 27For as many of you as have been baptized into Christ have put on Christ. 28There is neither Jew [or descendant of Judah] nor Greek, there is neither bond nor free, there is neither male nor female: for ye are all one in Christ Jesus. 29And if ye be Christ’s, then are ye Abraham’s seed, and heirs according to the promise.
16And as many as walk according to this rule, peace be on them, and mercy, and upon the Israel of God. 17From henceforth let no man trouble me: for I bear in my body the marks of the Lord Jesus. 18Brethren, the grace of our Lord Jesus Christ be with your spirit. Amen.
God will have mercy on those he chooses to have mercy. God chooses to have mercy on those who have faith in his Son, Jesus the Christ. All who calls on the name Jesus shall be saved! If you are in Christ, you are heirs according to the promise. The promise counts, not the flesh!