15 ¶ And God said unto Abraham, As for Sarai thy wife, thou shalt not call her name Sarai, but Sarah shall her name be. 16 And I will bless her, and give thee a son also of her: yea, I will bless her, and she shall be a mother of nations; kings of people shall be of her. 17 Then Abraham fell upon his face, and laughed, and said in his heart, Shall a child be born unto him that is an hundred years old? and shall Sarah, that is ninety years old, bear? 18 And Abraham said unto God, O that Ishmael might live before thee! 19 And God said, Sarah thy wife shall bear thee a son indeed; and thou shalt call his name Isaac: and I will establish my covenant with him for an everlasting covenant, and with his seed after him. 20 And as for Ishmael, I have heard thee: Behold, I have blessed him, and will make him fruitful, and will multiply him exceedingly; twelve princes shall he beget, and I will make him a great nation. 21 But my covenant will I establish with Isaac, which Sarah shall bear unto thee at this set time in the next year. 22 And he left off talking with him, and God went up from Abraham.
23 ¶ And Abraham took Ishmael his son, and all that were born in his house, and all that were bought with his money, every male among the men of Abraham’s house; and circumcised the flesh of their foreskin in the selfsame day, as God had said unto him. 24 And Abraham was ninety years old and nine, when he was circumcised in the flesh of his foreskin. 25 And Ishmael his son was thirteen years old, when he was circumcised in the flesh of his foreskin. 26 In the selfsame day was Abraham circumcised, and Ishmael his son. 27 And all the men of his house, born in the house, and bought with money of the stranger, were circumcised with him.
Sarai’s name is now changed to Sarah. She is part of the covenant, the promise. Not only will the son of promise come from Abraham but he will come through her, as well. She will be the mother of nations and kings of people will come from her.
The idea that a 90 year old woman, even in those ancient days when people lived much longer than they typically do now and were youthful much longer than they typically are now, would bear a child was preposterous. Abraham was so overtaken by the idea that he fell on his face and laughed in front of God! He said nothing about his astonishment out loud but then said aloud he hoped that Ishmael, who was already with him, would be the one promised to him.
The phrase said in his heart is used dozens of times to indicate private thoughts. We do not say, “thought in his brain,” when discussing things of a fundamentally private or emotional nature. We use phrases like, “my heart tells me,” or say that someone is kind-hearted.
God then insisted that Sarah would bear a son and that his name would be called Isaac, which means appropriately in the Hebrew, according to Strong’s dictionary, “He laughs.” God knows what you think as this is stated several times in the Bible from Genesis 6:5; 1Chronicles 28:9; Job 42:2; Psalm 94:11; Psalm 139:2; etc. Note that God stated he established His covenant with Isaac.
Ishmael will be taken care of and this is part of God’s grace and kindness in spite of the circumstances under which Ishmael was conceived. But, God reinforces that the covenant will be executed through Isaac. Abraham then takes Ishmael and all of the men of his household regardless of where they were from and circumcises them.
The word stranger is used to denote a foreigner, an alien to a people, who lives among that people.
Genesis 15:13 And he said unto Abram, Know of a surety that thy seed shall be a stranger in a land that is not theirs, and shall serve them; and they shall afflict them four hundred years;
Genesis 17:12 And he that is eight days old shall be circumcised among you, every man child in your generations, he that is born in the house, or bought with money of any stranger, which is not of thy seed.
Leviticus 24:22 Ye shall have one manner of law, as well for the stranger, as for one of your own country: for I am the LORD your God.