“Not as though the word of God hath taken none effect. For they are not all Israel, which are of Israel: neither, because they are the seed of Abraham, are they all children: but, ‘In Isaac shall thy seed be called.’
That 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. For this is the word of promise, At this time will I come, and Sara shall have a son.” Romans 9:6-9 (KJB)
Abraham had a son by Hagar, Ishmael (Genesis 16); but he was not God’s promised son. The promise was that Sarah would have a son. Many people do not realize it but Abraham had many other sons too through another wife by the name of Keturah (Genesis 25:1-6); so just because you may be born of the stock of Abraham does not make you a child of God.
You become a child of God through the Son of Promise. Isaac was a child born of faith to a man 100 years of age, and a woman of 90 years of age. God promised them a son. They had a son in God’s time; not theirs.
The Son of Promise through which one becomes a child of God is the Son of God Himself who born into this world as any other child, yet not just any other child – He is the Son of God. He came for one purpose, and that purpose was to glorify His Father; and He would do that by giving Himself as the perfect Lamb of God on a cross to die for the sins of the world.
Jesus Christ, the Son of God, God the Son died on the cross, was buried, and He rose again fulfilling all the Father’s will for His life. He is the Son of Promise. He is the way for anyone to become a child of God; by grace through faith call on His name. His name is Jesus.