Rom 9:9
9For
this is the word of promise, At this time will I come, and Sarah shall have a son.
Gal 4:28
28Now we, brethren, as Isaac was, are the children of promise.
Thats what it means to be a child of promise , a believer. Just as Isaac was.
God promised Abraham a believing seed.
Gen 17:2-5
1And when Abram was ninety years old and nine, the LORD appeared to Abram, and said unto him, I am the Almighty God; walk before me, and be thou perfect.
2And I will make my covenant between me and thee,
and will multiply thee exceedingly.
3And Abram fell on his face: and God talked with him, saying,
4As for me, behold, my covenant is with thee,
and thou shalt be a father of many nations.
5Neither shall thy name any more be called Abram, but thy name shall be Abraham; for a father of many nations have I made thee.
Each child of promise, God will reveal to them the Covenant as He did Isaac.
Look
Gen 26:1-4
1And there was a famine in the land, beside the first famine that was in the days of Abraham. And Isaac went unto Abimelech king of the Philistines unto Gerar.
2And the LORD appeared unto him, and said, Go not down into Egypt; dwell in the land which I shall tell thee of:
3Sojourn in this land, and I will be with thee, and will bless thee; for unto thee, and unto thy seed, I will give all these countries, and I will perform the oath which I sware unto Abraham thy father;
4And
I will make thy seed to multiply as the stars of heaven, and will give unto thy seed all these countries;
and in thy seed shall all the nations of the earth be blessed;
Thats the Gospel being revealed, Peter Called it the Covenant
Acts 3:25
25Ye are the children of the prophets,
and of the covenant which God made with our fathers, saying unto Abraham,
And in thy seed shall all the kindreds of the earth be blessed.
Paul calls the same Promise the Gospel
Gal 3:8
8And the scripture, foreseeing that God would justify the heathen through faith,
preached before the gospel unto Abraham, saying, In thee shall all nations be blessed.
So the Covenant and the Gospel are the same. Now all the Children of Promise, who are born sinners by nature as others are, and so was Isaac, shall in their lifetime have the Covenant or Gospel revealed to them because they are Children of the promise and when that happens they believe..
They are Children of the Covenant. What Covenant ? This one
Gen 17:2-5,
7
2And
I will make my covenant between me and thee, and will multiply thee exceedingly.
3And Abram fell on his face: and God talked with him, saying,
4As for me, behold, my covenant is with thee, and thou shalt be a father of many nations.
5Neither shall thy name any more be called Abram, but thy name shall be Abraham; for a father of many nations have I made thee.
7And
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.
So believers are a result of God establishing His Covenant with Abraham's seed, or Children after Him as He said.
So therefore Paul writes
Gal 3:7
7Know
ye therefore that they which are of faith, the same are
the children of Abraham.
Thats why they are of Faith, because of God's Promise to multiply Abraham's Seed.
They are a result of God's word of Promise ! BTW, That is who the True Israel of scripture is as well, The Children of Promise !
Rom 9:8