John Calvin Commentary Genesis 21:11

"And the thing was very grievous in Abraham`s sight on account of his son." — Genesis 21:11 (ASV)

And the thing was very grievous in Abraham’s sight. Although Abraham had already been assured by many oracles that the blessed seed should proceed from Isaac only, yet, under the influence of paternal affection, he could not bear for Ishmael to be cut off, so that the inheritance would remain entirely for Isaac, to whom it had been divinely granted. Thus, by mingling two races, he endeavored, as far as he was able, to confound the distinction which God had made.

It may indeed seem absurd that God's servant should be carried away like this by a blind impulse. But God deprives him of judgment in this way, not only to humble him but also to testify to all ages that the dispensing of his grace depends on his own will alone.

Moreover, so that the holy man might bear his son's departure with greater equanimity, he is promised a double consolation. For first, God reminds him of the promise made concerning Isaac, as if he would say, it is enough and more than enough, that Isaac, in whom the spiritual blessing remains complete, is left.

He then promises that he will take care of Ishmael, though exiled from his paternal home, and that a posterity will arise from him that will constitute a whole nation. But I have explained above, in the seventeenth chapter (Genesis 17:1), what is the meaning of the expression The seed shall be called in Isaac. And Paul (Romans 9:8), as an interpretation, uses the word reckoned or imputed. And it is certain that by this method, the other son was cut off from Abraham's family, so that he would no longer have a name among his posterity.

For God, by severing Ishmael, shows that the whole progeny of Abraham should flow from one head. He also promises Ishmael that he will be a nation, but estranged from the Church, so that the brothers' condition will, in this respect, be different: one is established as the father of a spiritual people, while the other is given a carnal seed.

From this, Paul justly infers that not all who are Abraham's seed are true and genuine sons, but only those who are born of the Spirit. For just as Isaac himself became the legitimate son by a gratuitous promise, so the same grace of God makes a distinction among his descendants. But because we have sufficiently discussed the various sons of Abraham in the seventeenth chapter, the subject is now alluded to more sparingly.