Genesis 18:10
And he said, I will certainly return unto thee according to the time of life; and, lo, Sarah thy wife shall have a son. And Sarah heard it in the tent door, which was behind him. And he said, I will certainly return unto thee according to the time of life; and, lo, Sarah thy wife shall have a son. And Sarah heard it in the tent door, which was behind him.
Heavenly angels came to Abraham with promises of blessing on their way to destroy Sodom, sent a second time on a mission of mercy that shows how angels in all ages minister to God's people. The promise they spoke was that Sarah would have a son.
It repeats more definitely and more distinctly God's earlier promise that Sarah herself would bear Abraham a son. It restates the promise given after the birth of Ishmael that Sarah would be mother of many nations and would bear Isaac, with whom and with his seed the everlasting covenant would be established. Abraham had not yet understood that promise, with his mind resting on Ishmael as the one through whom the many nations would come.
The promise confirms that the promised child would come through Sarah, not Ishmael. Isaac is the son whose birth fulfills it, bringing great joy to Abraham and Sarah and establishing him as the son of promise, as Sarah's son and heir, distinct from Ishmael.
God gave the promise to prove Abraham's firm faith and reliance on what was promised, meaning Abraham and Sarah themselves should have waited in confiding faith for its fulfillment even though Sarah was old. They believed God would keep His promise yet could not believe Sarah in her old age would have a son, which showed lack of faith and perfect trust in God's unlimited power. Taking Hagar as wife instead of waiting failed the test of faith and brought much unhappiness upon Abraham and upon Sarah.
Abraham remembered the heavenly messengers' promise that Sarah would have Isaac when he faced God's command to offer Isaac as a sacrifice. The family conflict that followed the fulfillment teaches that marriage rights and the happiness of the marriage relation are to be respected and guarded, even at great sacrifice.
The verse is to be prayed over to be made effectual in reaching hearts and to be used to call hearers to seek the Lord most earnestly and take a more decided stand for Him. It calls those who are only professionally believers in the truth to become obedient to all of God's requirements, with heartfelt confession and earnest prayer, while the Spirit touches, softens and subdues hearts.