Those of us who believe in Christ are Abraham's descendants, and we are the heirs of the covenant promises.
In Genesis 17, God said:
"As for Me, behold, My covenant is with you, And you will be the father of a multitude of nations." (Genesis 17:4 NASB)
Genesis makes it clear that Abraham did NOT father a multitude of nations in a physical OR political sense. The meaning of God's promise was probably that a multitude of nations would enjoy the blessings of sonship, even though they are not biologically related to Abraham.
There is no doubt of what God meant in Genesis 12:3 when he said:
"And I will bless those who bless you, And the one who curses you I will curse.And in you all the families of the earth will be blessed." (Genesis 12:3 NASB)From the beginning, God has known that Jesus would be the descendant of Abraham and that everyone who trusted in Him would be come an heir to Abraham's promise.
So, look at Galatians 3:29:
"And if you belong to Christ, then you are Abraham’s descendants, heirs according to promise." (Galatians 3:29 NASB)When God said "Behold, My covenant is with you, and you will be the father of a multitude of nations." He opened the way so that any of us could become a child of Abraham, and an heir to the promises God made to Abraham. All we have to do is share the faith that Abraham had -- beleive in God's promises so that we obey in such a way that we could do like Abraham when he was willing to give up Isaac. We aren't working for God, but we are confident that God works for us. Look at Romans 4 (speaking of Abraham):
"Yet, with respect to the promise of God, he did not waver in unbelief but grew strong in faith, giving glory to God" (Romans 4:20 NASB)THAT is why Abraham could obey God EVEN when obedience seemed crazy...he trusted in God to do what seemed impossible.
Faith in God's promises, faith in Jesus Christ (who is the confirmation of God's promises), is how we become a child of Abraham. Obeying God is the evidence that our faith is genuine. Let's look at another passage in Genesis:
He said, “Do not stretch out your hand against the lad, and do nothing to him; for now I know that you fear God, since you have not withheld your son, your only son, from Me.” Then Abraham raised his eyes and looked, and behold, behind him a ram caught in the thicket by his horns; and Abraham went and took the ram and offered him up for a burnt offering in the place of his son. Abraham called the name of that place The Lord Will Provide, as it is said to this day, “In the mount of the Lord it will be provided.”That is the ultimate in obedience!
Then the angel of the Lord called to Abraham a second time from heaven, and said, “By Myself I have sworn, declares the Lord, because you have done this thing and have not withheld your son, your only son, indeed I will greatly bless you, and I will greatly multiply your seed as the stars of the heavens and as the sand which is on the seashore; and your seed shall possess the gate of their enemies. In your seed all the nations of the earth shall be blessed, because you have obeyed My voice.” So Abraham returned to his young men, and they arose and went together to Beersheba; and Abraham lived at Beersheba. (Genesis 22:12-19 NASB)
They answered and said to Him, “Abraham is our father.” Jesus said to them, “If you are Abraham’s children, do the deeds of Abraham. (John 8:39 NASB)So, the children of Abraham are people of all nations, all over the world who put their faith in Jesus Christ and like Abraham did up on Mount Moriah don't let their most prized possessions get in the way of their obedience. Those of us who have faith in Jesus, follow Him and have faith and obedience are the Children of Abraham.