Abraham had eight sons: Ishmael from Hagar, Isaac from Sarah, and six sons — Zimran, Jokshan, Medan, Midian, Ishbak, and Shuah — all from Keturah, his second wife after Sarah’s death. There is a blessing for all of Abraham’s children, but the special, everlasting covenant is only for Isaac because this promise was made to Sarah’s son, and she had only one son.
Genesis 17:1-22 is God’s full promise to Abraham reiterated and fully detailed. In previous chapters, he hints at the rewards and makes a promise, but doesn’t give details about Abraham’s side of the covenant. In verses 1-6, he says to Abraham to “Serve me faithfully and live a blameless life”, meaning stop sinning and live righteously on purpose. This statement, along with what he says later about Ishmael receiving a blessing but not being the promised seed of the everlasting covenant, who would be Isaac. This implies that Abraham’s abuse of Hagar as a surrogate was a sin because he and Sarah doubted God’s abilities and acted on their own to create the promised child.
In addition, God changes Abram’s name (meaning great father) to Abraham (father of many nations). In verses 7-8, God presents His side of the covenant, which promises an everlasting covenant and grants the land of Canaan to all the nations that descend from Him. Then, in verses 9-14, God instructs Abraham on what he must do, which is to teach his descendants to worship only one God and to dedicate themselves to Him through male circumcision. Then in verses 15-16, God talks about Sarai’s role in the promised covenant and God changes her name to Sarah (meaning princess), and says she will be blessed as the mother of nations (Jacob/Israel & Esau/Edom).
In verse 17, Abraham laughs at the thought of Sarah and him having a natural child at such an old age (99 for Abraham and 89 for Sarah). Then in verse 18, he wishes that Ishmael could be in God’s presence. This further indicates that God had rejected Ishmael and the circumstances of his birth.
God replies in verse 19 that Sarah will have a child and HE will receive the EVERLASTING COVENANT, and his name will be Isaac, which means laughter.
In verses 20-21, God confirms that Ishmael is rejected as the promised seed that will inherit the everlasting covenant, but promises to give him and the 12 nations that will later come from him a blessing for being Abraham’s descendant. Verse 21 reiterates verse 19, saying that the everlasting covenant is only for Isaac because he is Sarah’s son.
This is reiterated in Genesis 21:8-13 when Sarah wants to kick Ishmael and Hagar out for bullying her son, Isaac, at the weaning ceremony. Abraham is troubled, but God reminds him that, despite the everlasting covenant with Sarah’s son, there is still a blessing for Ishmael. Later, in verses 14-21, God reveals this to Hagar and Ishmael as they wander in the desert, showing them an oasis and providing for them in the future. Eventually, they return to Egypt, Ishmael becomes a skilled hunter, and marries. He has 12 sons who will become the 12 tribes of Ishmael. Long story short, Isaac is the promised seed, not Ishmael, even though he was the firstborn, because the covenant is with Sarah, Isaac’s mother.