Genesis 21: We come now to the long-awaited birth of Isaac, concerning which the New Testament says, "By faith Sarah herself also received strength to conceive seed, and she bore a child when she was past the age, because she judged Him faithful who had promised. Therefore, from one man, and him as good as dead, were born as many as the stars in the sky in multitude—innumerable as the sand which is by the seashore" (Hebrews 11:11-12). While the author of Hebrews praises the faith of Sarah in this respect, the Apostle Paul tends rather to stress the faith of Abraham (Romans 4:19-22). The circumcision of Isaac (verse 4), commanded in Genesis 17:9-14), would be explicitly mentioned by St. Stephen in Acts 7:8.
In Genesis 16 we already learned that all was not well between Sarah and Hagar after Ishmael was born. At that time, however, Hagar enjoyed the advantage that she had borne a son, and Sarah had not. In the present chapter that advantage is a thing of the past, and we are not surprised to see that now Hagar and Ishmael are regarded as the mere slaves that they were. Ishmael is accused of "scoffing" at the younger child Isaac, perhaps a reference to the kinds of teasing that younger children have been known to suffer from older children. Indeed, one may reasonably speculate that Ishmael had heard disparaging remarks about Sarah and Isaac from his own mother and was simply acting them out. At the very least, Sarah does not want her son playing with a mere slave boy.
So Hagar must go. Ishmael's true situation is revealed in the fact that he is not even named; he is simply "that slave girl's son" (verse 10). In Sarah's eyes he has become a non-entity. Abraham is faced with a new problem, therefore. Although Ishmael is not Sarah's son except in a purely legal sense that no longer bore legal significance, the older boy is still Abraham's son, and Abraham loves him.
Whatever Sarah's reasons for expelling Hagar and Ishmael, God had His own reasons, and He permitted Sarah's plans to succeed in order for His own reasons to succeed. This is true rather often; God permits evil to prevail for the sake of a greater good that only He can see and plan for. Had Hagar and Ishmael stayed on in Abraham's household, they would have remained slaves. By their departure Ishmael was able to become the father of a great people on the earth (verse 13), a great people with us to this day, the great people of Arabia, for whom God manifested a special providential interest in this text. We will meet this theme of divine providence abundantly in the Joseph story toward the end of Genesis.
The biblical text tends to lose track of Hagar and Ishmael once they arrive in the Negev Desert. The legends of the Arabs tell their own story of how far the mother and child reached in their journey, namely, Mecca. The spring in verses 14-19 the Arabs identify as the spring of Zamzam, found near the Ka'ba at Mecca, which spring allowed human life to flourish in that place. Thus, Ishmael is credited with the founding of Mecca, which is a religious shrine vastly older than Islam.