"And the LORD visited Sarah as He had said, and the LORD did for Sarah as He had spoken. For Sarah conceived and bore Abraham a son in his old age, at the set time of which God had spoken to him. And Abraham called the name of his son who was born to him--whom Sarah bore to him--Isaac. Then Abraham circumcised his son Isaac when he was eight days old, as God had commanded him. Now Abraham was one hundred years old when his son Isaac was born to him. And Sarah said, 'God has made me laugh, and all who hear will laugh with me.' She also said, 'Who would have said to Abraham that Sarah would nurse children? For I have borne him a son in his old age'" (Gen. 21:1-7).
Approximately 25 years after Abraham had left his home, God gave him the promised son. Isaac's name means "laughter", which is quite significant in this case. Sarah was ecstatic at 90 years old! Clearly, God was involved in a special way to make this son a reality. Isaac was circumcised on the eighth day as God had instructed, as a sign of the covenant God had made with Abraham.
"So the child grew and was weaned. And Abraham made a great feast on the same day that Isaac was weaned. And Sarah saw the son of Hagar the Egyptian, whom she had borne to Abraham, scoffing. Therefore she said to Abraham, 'Cast out this bondwoman and her son; for the son of this bondwoman shall not be heir with my son, namely with Isaac.' And the matter was very displeasing in Abraham's sight because of his son" (Gen. 21:8-11).
Although it is rare for mothers today in America to nurse their babies beyond one year of age, it was common in Abraham's day for a mother to nurse a child for two or three years. Thus, Ishmael is a mid-teenager by this time, and Sarah does not like his attitude toward Isaac. She strongly desires for Hagar and Ishmael to leave, and she makes this wish known to Abraham. Sarah doesn't want Ishmael to be an heir alongside of her Isaac. Although Sarah's feelings are decisive, Abraham is torn. He loves both sons, of course. Here is a simple illustration of the disaster of polygamy (even in the home of a godly man). One father plus multiple mothers plus half-siblings is always a bad formula! One man plus one woman for life avoids a lot of problems; God's way is always best!
In the next section, God exhorts Abraham to listen to Sarah's council. It mattered not whether Ishmael stayed near Abraham while growing up. God would take care of him and make him into a great nation for the sake of Abraham. The promises would be fulfilled through Isaac's lineage, but Ishmael would be blessed also. So Abraham listened to God and Sarah and sent Hagar and Ishmael on their way, after providing them with some supplies. Unfortunately, the water was used up before they reached their destination. Hagar believed she and her son would die, and she wept loudly. God heard her voice and responded favorably with help. Water was provided and Ishmael would dwell in the wilderness. He became an archer and took an Egyptian wife.
The closing section of Genesis 21 details a covenant made between Abraham and Abimelech. This is the same Abimelech whom Abraham had lied to in Genesis 20. Abimelech is aware of the fact that God is with Abraham and has blessed him richly. He wants Abraham to swear that he would not deal treacherously with him or his descendants. Abimelech fears Abraham, and the great patriarch is willing to make the oath. Some have suggested that Abimelech may have been more drawn to Sarah initially for reasons of making an ally of Abraham, rather than for her beauty. Regardless, the king is now actively seeking peace with this rich and powerful patriarch. Abraham, however, has reason to rebuke Abimelech since his men had seized a well from Abraham. The two made a covenant together and Abraham included seven ewe lambs as witness that he had dug the well in question (it appears that Abimelech returned it to him). The men then depart in peace. "Then Abraham planted a tamarisk tree in Beersheba, and there called on the name of the LORD, the Everlasting God. And Abraham stayed in the land of the Philistines many days" (Gen. 21:33,34).