Summary: God chose that through Isaac and not Ishmael that Christ would be born.
The Life of Abraham, Part 14: Isaac Shall Be Heir
The birth of Isaac was a time of great joy. But the birth of Isaac also would have important implications. I would suppose that Hagar and Ishmael rejoiced in the birth as well, at least outwardly. But is would not be long before their standing in Abraham’s household will be challenged.
Exposition of the Text
It took a couple of years for the conflict to break out. But Isaac had reached an important milestone in his life. He was no longer dependent upon his mother’s milk. He had made the first step of a long journey unto adulthood. So Abraham prepared a great feast for all his household to rejoice. At this time Ishmael was about fifteen and would have been considered as an adult in that society or very nearly so. There was no great rejoicing recorded for Ishmael for his having achieved an important milestone in his life.
At the feast, it is said that Ishmael was either joking with the young lad or more likely mocking him. Sibling rivalries seem to be the rule and not the exception in human society. We also know that older children have trouble adjusting to not being the only child any longer. They tend to feel neglected as more attention has to be focused upon a totally dependent child. All of this would ring true enough for Ishmael, but there was even a deeper rift felt by the boy. He was the son of a slave woman. Even though he was the older son, Isaac was the one who had higher legal status in the family. The son of a slave woman was still a slave, no matter who his father was. Ishmael may have felt this deeply. The prophecy given to Hagar was that the boy was going to struggle with everyone. His hand would be against them and theirs against his.
There could be another way to look at this in that Ishmael may only have been playing with Isaac. In that case, it was a problem with Sarah and not Ishmael. The NET Bible text indicates the possibility that Sarah took the slave boy’s playing with Isaac as making Ishmael a child of equal or even superior status which threatened her son inheriting fully the estate and the promise. The context makes either of these a possibility, so one should be careful not to jump to conclusions as who was blameworthy. The important detail is that the conflict over who would be Abraham’s heir had come to the surface, and it had to be resolved.
Sarah seems to have been a woman of some temper, especially when her interests were crossed. She stormed to Abraham and demanded that the slave woman and her son be banished from Abraham’s household. She did not care how precarious it made their situation in society. Where would they go and how they might make a living was irrelevant. The only thing that mattered is that all rivals to her son’s inheritance be removed.
Abraham naturally hotly resisted the idea. After all, even though Ishmael was not Sarah’s son, he was Abraham’s son. Abraham was concerned for his welfare. He may have had an idea that the banishment of Ishmael was a death sentence, and it very nearly was. A rift was developing between Abraham and Sarah over this which threatened the unity of the household.