9 For this is the word of promise: “At this time I will come and Sarah shall have a son.”
Verse nine functions as the scriptural proof of verses seven and eight. This establishes that genuine believers are so because of God’s promise and not their lineage.
The “for” shows that it wasn’t Abraham’s physical children that are His regenerate children but only those who believe His promise of grace.
this is the word of promise [emphatic]:
Having said that the Word of God does not fail (v. 6), Paul now showed emphatically that God’s promise did not fail. God is always faithful to His promise. Paul quoted freely another verse from the Old Testament (Septuagint): Genesis 18:10. This confirms that God is just in what He does.
“At this time I will come and Sarah shall have a son.”
Now Paul took up the nature of the promise. The quote here elaborates on God’s promise. We receive what we receive from God by promise, not by human privilege or physical inheritance. God regenerates through promise, not works or religious heritage. Salvation is always from God’s initiative. God’s choice of Isaac over Ishmael demonstrates the principle of promise in salvation.
God’s promises cannot fail because they rest on His call, which is guaranteed.
God’s promise is invincible because it rests on His Son who came through Sarah. The birth of Isaac was a work of God in fulfilling the Abrahamic contract (covenant). God is just in all that He does.
Even the first Jew himself, Abraham, came to regeneration through faith:
Ge 15:6, And he believed in the Lord, and He accounted it to him for righteousness.