John Calvin Commentary Genesis 21:1

"And Jehovah visited Sarah as he had said, and Jehovah did unto Sarah as he had spoken." — Genesis 21:1 (ASV)

And the Lord visited Sarah. In this chapter not only is the nativity of Isaac related, but because, in his very birth, God has set before us a vivid picture of His Church, Moses also gives a particular account of this matter. And first, he says that God visited Sarah, as He had promised. Because all offspring flows from the kindness of God, as it is in the psalm,

The fruit of the womb is the gift of God; (Psalms 127:3)

Therefore, the Lord is said, not without reason, to visit those to whom He gives children. For although the fetus seems to be produced naturally, each from its own kind, there is yet no fecundity in animals, unless the Lord puts forth His own power to fulfill what He has said, Increase and multiply.

But in the propagation of the human race, His special benediction is conspicuous; and therefore, the birth of every child is rightly deemed the effect of divine visitation. However, Moses, in this place, looks higher, since Isaac was born outside the accustomed course of nature.

Therefore, Moses here commends that secret and unusual power of God, which is superior to the law of nature. And not improperly, since it is of great consequence for us to know that the gratuitous kindness of God reigned, both in the origin and in the progress of the Church, and that the sons of God were born in no other way than from His mere mercy.

And this is the reason why He did not make Abraham a father until his body was nearly withered. It should also be noted that Moses declares the visitation he mentions to be founded upon promise: Jehovah visited Sarah, as He had promised. In these words he connects the effect to its cause, so that the special grace of God, of which an example is given in the birth of Isaac, might be more perceptible.

If he had merely said that the Lord showed regard for Sarah when she brought forth a son, some other cause might have been sought. None, however, can doubt that the promise by which Isaac had been granted to his father Abraham was gratuitous, since the child was the fruit of that adoption, which can be ascribed to nothing but the mere grace of God. Therefore, whoever wishes rightly and prudently to reflect upon the work of God in the birth of Isaac must necessarily begin with the promise.

There is also great emphasis in the repetition, The Lord did unto Sarah as He had spoken. For he thus retains his readers, as if laying his hand upon them, so that they may pause in considering so great a miracle. Meanwhile, Moses commends the faithfulness of God, as if he had said: God never feeds people with empty promises, nor is He less true in granting what He has promised than He is liberal and willing in making the promise.