The Lord did for Sarah as he promised, she became pregnant at God’s appointed time – Isaac. Abraham was 100. Sarah saw Isaac mocking Ishmael (son of Hagar the Egyptian), and asked Abraham to drive them out, because Ishmael will not be a coheir with Isaac. This was hard for Abraham, but God told him to not be concerned for them, for He will make Ishmael into a nation as well. Abraham gave them provisions and sent them away. They ran out of provisions, and the Lord heard Ishmael’s cry, and the angel of God called her, and promised to make Ishmael a great nation and she saw a well. God was with Ishmael, and he grew; he settled in the wilderness, became an archer, and married an Egyptian. Abimelech and Phicol (the army commander) came to Abraham from the land of the Philistines and they made a covenant swearing an oath not to come against each other, and that Abraham dug a certain well at Beer-sheba. Abraham planted a tamarisk tree in Beer-sheba and called on the name of Yahweh, the Everlasting God, and he lived in the land of the Philistines for many days.
God’s promise of a child to Abraham and Sarah is fulfilled. Here we see the dividing of nations – Isaac & Ishmael. God fulfills his promise to Hagar that Ishmael will be a great nation, but this would not be God’s chosen line to extend His covenant through. It is through the line of Isaac that God continues to work to bring a Savior who will crush the head of the serpent in fulfillment of Genesis 3:15. Also, Abraham’s line is already becoming a nation. This is why Abraham and the Philistines make a covenant not to come against each other. God is fulfilling His promises.