Wherefore she said unto Abraham, Cast out this bondwoman and her son: for the son of this bondwoman shall not be heir with my son, even with Isaac.

Wesley's Notes

21:10 Cast out the bond - woman - This was a type of the rejection of the unbelieving Jews, who, though they were the seed of Abraham, yet, because they submitted not to the gospel - covenant, were unchurched and disfranchised. And that, which above any thing provoked God to cast them off, was, their mocking and persecuting the gospel - church, God's Isaac, in his infancy.

Jamieson-Fausset-Brown Bible Commentary

10. Wherefore she said unto Abraham, Cast out this bondwoman-Nothing but the expulsion of both could now preserve harmony in the household. Abraham's perplexity was relieved by an announcement of the divine will, which in everything, however painful to flesh and blood, all who fear God and are walking in His ways will, like him, promptly obey. This story, as the apostle tells us, in "an allegory" [Ga 4:24], and the "persecution" by the son of the Egyptian was the commencement of the four hundred years' affliction of Abraham's seed by the Egyptians.

Matthew Henry's Concise Commentary

21:9-13 Let us not overlook the manner in which this family matter instructs us not to rest in outward privileges, or in our own doings. And let us seek the blessings of the new covenant by faith in its Divine Surety. Ishmael's conduct was persecution, being done in profane contempt of the covenant and promise, and with malice against Isaac. God takes notice of what children say and do in their play; and will reckon with them, if they say or do amiss, though their parents do not. Mocking is a great sin, and very provoking to God. And the children of promise must expect to be mocked. Abraham was grieved that Ishmael should misbehave, and Sarah demand so severe a punishment. But God showed him that Isaac must be the father of the promised Seed; therefore, send Ishmael away, lest he corrupt the manners, or try to take the rights of Isaac. The covenant seed of Abraham must be a people by themselves, not mingled with those who were out of covenant: Sarah little thought of this; but God turned aright what she said.