"God also said to Abraham, 'As for Sarai your wife, you are no longer to call her Sarai; her name will be Sarah. I will bless her and will surely give you a son by her. I will bless her so that she will be the mother of nations; kings of peoples will come from her.'" NIV translation
God changed Sarai's name to Sarah and we are reminded that, when God changes a person's name, it is a reminder that they have become His children. He renews His promise to provide sons to carry on the covenant.
"Abraham fell facedown; he laughed and said to himself, 'Will a son be born to a man a hundred years old? Will Sarah bear a child at the age of ninety?' And Abraham said to God, 'If only Ishmael might live under your blessing!'" NIV translation
We notice that Abraham fell "facedown" as God spoke to him which is the normal reaction for men. Then, he laughed in unbelief as he could not get past the physical limitations of man. In his unbelief, he asked God to bless Ishmael instead of trusting in God's promise. This unbelief has built a lasting legacy as even today the descendants of Ishmael reject the promise of God which was made known through His Son, Jesus Christ.
"Then God said, 'Yes, but your wife Sarah will bear you a son, and you will call him Isaac. I will establish my covenant with him as an everlasting covenant for his descendants after him. And as for Ishmael, I have heard you: I will surely bless him; I will make him fruitful and will greatly increase his numbers. He will be the father of twelve rulers, and I will make him into a great nation. But my covenant I will establish with Isaac, who Sarah will bear to you by this time next year.' When he had finished speaking with Abraham, God went up from him." NIV translation
God agreed to bless Ishmael meaning he would not have to worry about producing a son like Abram had worried.
Instead, God promised that he would be the father of twelve rulers and a great nation. This "great nation" speaks of the fact that they would be great in number but not great in pleasing God. This "great nation" was speaking of those who follow the teachings of Islam.
Although God agreed to bless Ishmael, He made it clear that the descendants of Isaac would be known as His people. (Ishmael would be the father of a great nation but not the father of the promised nation.)
This was said twice in this passage and we must remember that, throughout the Bible, the number two is associated with truth as matters were established on the word of two witnesses.
"On that very day Abraham took his son Ishmael and all those born in his household or bought with his money, every male in his household, and circumcised them, as God told him. Abraham was ninety-nine years old when he was circumcised, and his son Ishmael was thirteen; Abraham and his son Ishmael were both circumcised on that same day. And every male in Abraham's household, including those born in his household or bought from a foreigner, was circumcised with him." NIV translation
We see that Abraham was obedient and all the males in his household were circumcised. This confirmed the covenant and was a physical sign to the world of God's giving them the land of Canaan and being their God. This was a picture of the covenant of Christ that was to come through His blood and the circumcision of men's hearts.