15 Then God said to Abraham, "Regarding Sarai, your wife-her name will no longer be Sarai. From now on her name will be Sarah. 16 And I will bless her and give you a son from her! Yes, I will bless her richly, and she will become the mother of many nations. Kings of nations will be among her descendants." 17 Then Abraham bowed down to the ground, but he laughed to himself in disbelief. "How could I become a father at the age of 100?" he thought. "And how can Sarah have a baby when she is ninety years old?" 18 So Abraham said to God, "May Ishmael live under your special blessing!" 19 But God replied, "No-Sarah, your wife, will give birth to a son for you. You will name him Isaac, and I will confirm my covenant with him and his descendants as an everlasting covenant. 20 As for Ishmael, I will bless him also, just as you have asked. I will make him extremely fruitful and multiply his descendants. He will become the father of twelve princes, and I will make him a great nation. 21 But my covenant will be confirmed with Isaac, who will be born to you and Sarah about this time next year." 22 When God had finished speaking, he left Abraham.
Matthew Henry's Commentary on Genesis 17:15-22
Commentary on Genesis 17:15-22
(Read Genesis 17:15-22)
Here is the promise made to Abraham of a son by Sarai, in whom the promise made to him should be fulfilled. The assurance of this promise was the change of Sarai's name into Sarah. Sarai signifies my princess, as if her honour were confined to one family only; Sarah signifies a princess. The more favours God confers upon us, the more low we should be in our own eyes. Abraham showed great joy; he laughed, it was a laughter of delight, not of distrust. Now it was that Abraham rejoiced to see Christ's day; now he saw it and was glad, John 8:56. Abraham, dreading lest Ishmael should be abandoned and forsaken of God, put up a petition on his behalf. God gives us leave in prayer to be particular in making known our requests. Whatever is our care and fear, should be spread before God in prayer. It is the duty of parents to pray for their children, and the great thing we should desire is, that they may be kept in covenant with Him, and may have grace to walk before him in uprightness. Common blessings are secured to Ishmael. Outward good things are often given to those children of godly parents who are born after the flesh, for their parents' sake. Covenant blessings are reserved for Isaac, and appropriated to him.