Genesis 17:15-27

God changes Sarai's name to Sarah

15 Then God said to Abraham, ‘As for your wife, do not call her Sarai any more. Instead, her name will be Sarah. 16 I will bless her. She will give birth to a son for you. Many nations of people will be her descendants. Some of them will become kings. That is how much I will bless her!’

17 Abraham fell down so that his face touched the ground. He laughed and he said to himself, ‘I am now 100 years old. That is too old to have a son. Sarah is 90 years old, so she could never give birth to a child. That is too old to have a child.’ 18 So Abraham said to God, ‘I would be happy if you would bless Ishmael as my son.’

19 God said to Abraham, ‘No, your wife Sarah will give birth to a son for you. You must give to him the name “Isaac”. I will make my covenant with him and with his descendants for all time. 20 I have heard what you want me to do for Ishmael. I will bless him. I will cause him to have many descendants. Among his descendants there will be 12 rulers. His descendants will become a great nation. 21 But I will make my covenant with Isaac, not with Ishmael. At about this time next year, Sarah will give birth to your son, Isaac.’

17:19God tells Abraham that Sarah's child will continue Abraham's family line. It will not be Hagar's child, Ishmael. The name ‘Isaac’ means ‘he laughs’.
17:21God agrees to bless Ishmael because Abraham asked him to do that. But God makes his covenant with Isaac because he will be Abraham and Sarah's own son. That is what God had promised to do for them.

22 When God had finished speaking to Abraham, he went away.

23 On that same day, Abraham circumcised Ishmael and every male person in his house. He circumcised every male servant that was born in his house, as well as those that he had bought with money. He did this in the way that God had told him. 24 Abraham himself was circumcised when he was 99 years old. 25 His son, Ishmael, was 13 years old when he was circumcised. 26 Abraham and Ishmael were circumcised on the same day. 27 As well as them, all the male people living in Abraham's house were circumcised. This included the male servants that had been born in his house and those that he had bought with money.