And the thing was very grievous in Abraham's sight. Although Abraham had been already assured, by many oracles, that the blessed seed should proceed from Isaac only; yet, under the influence of paternal affection, he could not bear that Ishmael should be cut off, for the purpose of causing the inheritance to remain entire to him, to whom it had been divinely granted; and thus, by mingling two races, he endeavored, as far as he was able, to confound the distinction which God had made. It may truly seem absurd, that the servant of God should thus be carried away by a blind impulse: but God thus deprives him of judgment, not only to humble him, but also to testify to all ages, that the dispensing of his grace depends upon his own will alone. Moreover, in order that the holy man may bear, with greater equanimity, the departure of his son, a double consolation is promised him. For, first, God recalls to his memory the promise made concerning Isaac; as if he would say, it is enough and more than enough, that Isaac, in whom the spiritual benediction remains entire, is left. He then promises that he will take care of Ishmael, though exiled from his paternal home; and that a posterity shall arise from him which shall constitute a whole nation. But I have explained above, on 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,) by way of interpretation, uses the word reckoned, or imputed. [1] And it is certain that, by this method, the other son was cut off from the family of Abraham; so that he should no more have a name among his posterity. For God, having severed Ishmael, shows that the whole progeny of Abraham should flow from one head. He promises also to Ishmael, that he shall be a nations but estranged from the Church; so that the condition of the brothers shall, in this respect, be different; that one is constituted the father of a spiritual people, to the other is given a carnal seed. Whence Paul justly infers, that not all who are the seed of Abraham are true and genuine sons; but they only who are born of the Spirit. For as Isaac himself became the legitimate son by a gratuitous promise, so the same grace of God makes a difference among his descendants. But because we have sufficiently treated of the various sons of Abraham on the seventeenth chapter, the subject is now more sparingly alluded to.

Footnotes

1 - "Ponit verbum logizesthai, hoc est, censeri vel reputari."

And the thing was very grievous in Abraham's sight,.... The motion that Sarah made to turn out of his house Hagar and Ishmael was not agreeable to him, but the reverse; it seemed an ill thing to him; it was greatly displeasing to him, and he was unwilling to come into it:
because of his son; his son Ishmael; not grieved and uneasy for what he had done; not for committing idolatry, as the Targum of Jonathan suggests, or for mocking at Isaac; but for what was proposed to be done to him, the ejection of him from his house, because of the great love he had for him, and the great concern he had for his education, and that he might enjoy the blessing promised him, he, was loath to have him cast out of his family: no concern is expressed for Hagar, though both by what God said to Abraham, and by the provision he made for her, he had a regard unto her; but his chief concern was for his son, who perhaps had a greater share in his natural affections than as yet Isaac had; nor did express so much reluctance when he was bid to him up, as he did at this time, that being at the command of God, this at the instance of his wife, and which he supposed only proceeded from passion and resentment: the Hebrew writers say (x), that of all the evils that came upon Abraham this was the hardest and most grievous in his sight.
(x) Pirke Eliezer, c. 30.

The thing was very grievous in Abraham's sight - it grieved him that Ishmael had given such provocation. And still more that Sarah insisted upon such a punishment.