Genesis 21:6

In our father Isaac or the origin is sufficiently expressed, either by grace or by glory: to whom all glory redounds, that he was born to Abraham, a reward for such a worthy and admirable man. Nor is it surprising, since he preceded in the figure of the Lord's generation and passion. For even a barren old woman gave birth to him according to God's promise, so that we may believe that God is able to make a virgin conceive and that the only begotten one offered as a sacrifice would not perish for the father and would fulfill the sacrifice. Therefore, by its very name, it signifies both form and grace. For Isaac, in Latin, means laughter, and laughter is a notable sign of joy. And who is ignorant that he is the joy of all, who, with dreadful death or fear suppressed or sorrow removed, becomes the remission of sins for all? Therefore, he was named, and he was designated: he was expressed, and he was announced.

Isaac is interpreted as 'laughter' or 'joy'. Who then is he who begets such a son? Surely that one who said of those whom he had begotten through the gospel: "for you are my joy and my crown of boasting." Over sons of this sort, when they have been weaned from milk, there is a feast and great gladness—over those who "no longer need milk, but strong food, who by reason of their capacity to receive it have their senses exercised to discern good or evil." Over such as these, when they are weaned from milk, a great feast is held. But over those a feast cannot be presented nor gladness had, of whom the Apostle says: "I gave you milk to drink, not meat; for you were not yet able, but neither indeed are you able even now. And I could not speak to you as to spiritual, but as to carnal, as to little ones in Christ." Let those tell us, who wish the divine scriptures to be understood simply, what this means: "I could not speak to you as to spiritual, but as to carnal, as to little ones in Christ; I gave you milk to drink, not meat"? Can these things be taken simply?

Come now, today too, dearly beloved, let us take up the thread of yesterday's remarks and thus set before you this spiritual meal so that we may once more come to learn, as you heard yesterday, the good God's ineffable care and considerateness and the patriarch's remarkable obedience and gratitude. Do you see how the birth of Isaac made Sarah joyful? "She said, 'The Lord brought laughter to me: whoever hears of it will rejoice with me.' " Everyone who hears of it, she is saying, I will convince to be a sharer of my joy. After all, the gift given me by God is wonderful, surpassing human limitations. I mean, who would not be struck, she is saying, to see me feeding and nursing a child in old age after being childless up to this stage of my life? As though surprised and amazed at the event, she added, "Who will let Abraham know that Sarah is nursing a child, that I have borne a son in my old age?" Since what happened was beyond the bounds of nature, she naturally demands, "Who will let him know?" as if to say, Who will imagine it? Who would entertain such an idea? What mind could grasp it? Is there any reasoning that could invent this happening in every detail? Not so remarkable was the incident of the flood of water gushing from the rock in the desert when Moses struck it with his rod as was this instance of a child being born of a womb already deprived of vitality and a ready flow of milk. You see, for the birth to be known to everyone and bring everyone to acceptance of the marvel, for those who heard of it both then and later, she feeds the child and insists on nursing it. She said, "Who will let Abraham know that Sarah is nursing a child, that I have borne a son in my old age," this strange, surprising favor done me in my old age? What is the meaning of "I have borne a son in my old age"? That even without sterility my time of life was sufficient to make me despair of bearing children. But all these obstacles the Lord caused to disappear and has granted me the birth of the child and the flow of milk.