Abraham was called the father of many nations by God in Genesis 17:1-6
When Abram was ninety-nine years old, the Lord appeared to him and said, “I am God Almighty; walk before me faithfully and be blameless. Then I will make my covenant between me and you and will greatly increase your numbers."
Abram fell facedown, and God said to him, “As for me, this is my covenant with you: You will be the father of many nations. No longer will you be called Abram; your name will be Abraham, for I have made you a father of many nations. I will make you very fruitful; I will make nations of you, and kings will come from you."
The nations referred to are those who came out of his biological son of promise, with Sarah, Isaac, (the Israelites from the 12 tribes, who were originally the 12 sons) from his biological son Ishmael, with Hagar, (the Ishmaelites, 12 princes of a nation God said in Genesis he would bless Ishmael and bring these princes and nation out of him (Arabs today) Genesis 17:20 NIV
And as for Ishmael, I have heard you: I will surely bless him; I will make him fruitful and will greatly increase his numbers. He will be the father of twelve rulers, and I will make him into a great nation.
, but also the New Nation;
Mathew 21:43 NIV
"Therefore I tell you that the kingdom of God will be taken away from you and given to a people who will produce its fruit."
which God said He would graft into Israel of old, Romans 11:17 NIV
'If some of the branches have been broken off, and you, though a wild olive shoot, have been grafted in among the others and now share in the nourishing sap from the olive root,')
who are also called the 'seed of Abraham', that Paul wrote about "If you are Christ's, then are you Abraham's seed, and heirs according to the promise." Gal 3:29
This nation is the 'Israel of God'*, that Paul identified, who are sons of promise, brought forth through Christ who came through Abraham and through the Israelites, and as they are grafted in they become the 'one new man'.
One New Man
Ephesians 2:11-22 One in Christ
Therefore remember that at one time you Gentiles in the flesh, called “the uncircumcision” by what is called the circumcision, which is made in the flesh by hands— remember that you were at that time separated from Christ, alienated from the commonwealth of Israel and strangers to the covenants of promise, having no hope and without God in the world. But now in Christ Jesus you who once were far off have been brought near by the blood of Christ. For he himself is our peace, who has made us both one and has broken down in his flesh the dividing wall of hostility by abolishing the law of commandments expressed in ordinances, that he might create in himself one new man in place of the two, so making peace, and might reconcile us both to God in one body through the cross, thereby killing the hostility. And he came and preached peace to you who were far off and peace to those who were near. For through him we both have access in one Spirit to the Father. So then you are no longer strangers and aliens, but you are fellow citizens with the saints and members of the household of God, built on the foundation of the apostles and prophets, Christ Jesus himself being the cornerstone, in whom the whole structure, being joined together, grows into a holy temple in the Lord. In him you also are being built together into a dwelling place for God by the Spirit.
(in this group would be all the 'sheep nations of Jesus' illustration of the end of time and the judgment.)
Israel of God would also include those who have gone before who were the true believers in God, God approved, those from the old covenant Israel.
But far be it from me to boast, save in the cross of our Lord Jesus Christ, through which the world hath been crucified unto me, and I unto the world. For neither is circumcision anything, nor uncircumcision, but a new creation. And as many as shall walk by this rule, peace be upon them, and mercy, and upon the Israel of God.