Let's start with the interesting way Paul talks about offspring. It's a recap from last week.
In verse 16, Paul clarifies what has been given to us and by whom:
Now the promises were made to Abraham and to his offspring. It does not say, “And to offsprings,” referring to many, but referring to one, “And to your offspring,” who is Christ.
Don't misunderstand. Paul well knew that offspring was a collective noun. He knew that when God promised Abraham many offspring, and that they would be as numerous as the sand and stars, the promise was collective. But Paul also knew that the promise of many offspring would come from one - Isaac, Abraham's firstborn. Out of one comes many, but the many have to start with one.
Here's what Paul is driving at, in a very clever way.
Paul knew the plural application of offspring. He knew God's promise to Abraham in Genesis to make him the father of many nations. He knew all that. But here's what else Paul knew in the chiseling down of the word offspring. The promise to Abraham would come through his son, Isaac. God chose Isaac, not Ishmael, who was Abraham's first son, through his wife's handmaiden, Hagar. Isaac was the son of the promise, through Sarah and Abraham. Isaac was the one chosen…elected…by God as the next in line to give birth to many nations. Ishmael wasn't abandoned by God. Ishmael received God's blessing, but he was not the son of the promise. God chose…God's electing grace…Isaac for that. And so Paul says Jesus would be the child of the promise to Abraham and Sarah.