Thirteen years pass. Abraham is 99. Sarah is 90. Both of them received their new names, the names we know them by. Abraham means father of a multitude. Sarah means princess.
God promises that they will be the parents of nations, of kings, of a multitude of people. God says that Sarah will give birth to the promised child. Abraham laughs. He is still questioning God. He still thinks Ishmael was a good idea and asks God to just use Ishmael to fulfill the promise.
Ishmael must have been Abraham’s pride and joy, his only child. Abraham believed for those thirteen years that this was the promised descendant. God reaffirms that He will make Ishmael into a great nation but then confirms that the promised child will be Isaac and that Isaac will inherit the blessing of the land. God repeats his promise and sets up a covenant, a firm promise, a binding promise. It is called an eternal, an everlasting covenant. This is for keeps and forever. This is the covenant spoken by God to Abraham, vs. 8-14.
I will give to you and to your offspring after you the land of your sojournings, all the land of Canaan, for an everlasting possession, and I will be their God. This is my covenant, which you shall keep, between me and you and your offspring after you: Every male among you shall be circumcised. It shall be a sign of the covenant between me and you. He who is eight days old among you shall be circumcised. Any uncircumcised male who is not circumcised in the flesh of his foreskin shall be cut off from his people; he has broken my covenant.
Despite his laughing at the promise, Abraham listens and obeys. Abraham is circumcised at ninety-nine. Ishmael is circumcised at thirteen. The new baby, Isaac, will be circumcised according to the covenant at eight days old.
In the New Testament, we get a new definition of circumcision, and of a Jew. Romans 2:29 says, “But a Jew is one inwardly, and circumcision is a matter of the heart, by the Spirit, not by the letter.” That’s how we get to become Jews, get to become part of Israel, by the circumcision of our hearts, by the Spirit of God cutting out our hearts of flesh and giving us the life of Christ. That’s how Arabs – who are descendants of Ishmael – get to become part of Israel, children of the promise. They can accept the life of Christ, the circumcision of their hearts, just like it was offered to the Jews.