For this is the word of promise,
At this time will I come, and Sarah shall have a son.
Read Chapter 9
Ambrosiaster
AD 400
This prefigures Christ, because Christ was promised to Abraham as a future son, in whom the word of the promise would be fulfilled. Commentary on Paul’s Epistles.
To prove that the children of Abraham are the children of the promise, he adduces the passage of Scripture: "I will come in a year's time, and Sara shall have a son. "(Genesis xviii.) This promised child was Isaac, the true son of the promise of God, and of the faith of Abraham; and not the son of the flesh, for Ismael was this as well as Isaac; but he was granted to the prayers of Abraham, a child of the grace and mercy of God. All the faithful, therefore, of whatever race or nation they may be, are in this sense the children of Abraham, by being gratuitously chosen by God, and by the fidelity in which they are imitators of Abraham. (Calmet)
It is not the children of the flesh who are children of God, but rather even in nature regeneration through baptism from above was sketched out beforehand…. For Sarah’s womb was colder than any water, owing to barrenness and old age. … And just as in her case it happened when her age was past hope, so in this case also it was when the old age of sins had come upon us that Isaac suddenly sprang up in youth, and we all became the children of God and the seed of Abraham.
