Bible passage: Genesis 21:12
Written: As early as 1400 BC
In Gen. 17:18-19, God gives Abraham a prophecy that his wife, Sarah, who was 90-years old, would give birth to her first child, a son who would be named Isaac, who would inherit the covenant that was promised to Abraham.
A few chapters later, in the prophecy of Gen. 21:12, after the birth of Isaac, that promise is reiterated and reinforced:
But God said to him, "Do not be so distressed about the boy and your slave woman. Listen to whatever Sarah tells you, because it is through Isaac that your offspring will be reckoned." (Genesis 21:12, NIV)
As was the case in Gen. 17:18-19, God is telling Abraham that his covenant would be inherited by Abraham's descendants through Isaac, not Ishmael, the son that Abraham had through Sarah's handmade, Hagar.
The nature and details of the covenant becomes clearer in later prophecies of the Old Testament, and it becomes clearer that it involves the Messiah. But for now, in this part of Genesis, which is the first book of the Bible, all we know is that something of Biblical importance is going to happen with the descendants of Abraham, through his son Isaac.
The New Testament, which was written about 2,000 years after the time of Abraham, alludes to Gen. 21:12 in the books or "epistles" of Hebrews and Romans:
even though God had said to him, "It is through Isaac that your offspring will be reckoned." (Hebrews 11:18, NIV)
Nor because they are his descendants are they all Abraham's children. On the contrary, "It is through Isaac that your offspring will be reckoned." In other words, it is not the children by physical descent who are God's children, but it is the children of the promise who are regarded as Abraham’s offspring. (Romans 9:7-8, NIV)
Both epistles are generally attributed to the Paul, the evangelist, who lived about 2,000 years ago.
Isaac's birth was a miracle in that Sarah was 90-years old at that time (Gen. 17:17), and well beyond the normal point of being able to bear children. And she had never children before the birth of Isaac. In fact the word "Isaac" means "he laughs," which is the reaction that Abraham had when God told him that his aging wife would one day give birth to a child (Gen. 17:17).
The birth of Isaac represents a miracle and a promise that God made and kept to Abraham, because of Abraham's faith in God. That is why the writer of the book of Romans refers to "children of the promise" - the believers of Jesus as the Messiah - as being true descendants of Abraham, because of their faith.