Genesis 21:4

Moses is read to us in the church. Let us beseech the Lord that, according to the word of the Apostle, it may not be that even among us, "when Moses is read, a veil is placed over our heart." For it was read that Abraham begot a son, Isaac, when he was a hundred years old. "And Sarah said," it says, "who will announce to Abraham that Sarah is nursing a boy?" "And then," it says, "Abraham circumcised the boy on the eighth day." Abraham does not celebrate this boy's birthday, but he celebrates the day of his weaning from milk, "and he makes a great feast." What then? Do we suppose that it was the Holy Spirit's purpose to write and narrate histories, how a boy was weaned from milk and a feast was held, how he played and did the other things children do? Or is it rather to be thought that through these things He wishes to lead us to something divine and worthy for the human race to learn from the words of God?

And he circumcised him on the eighth day, as God had commanded him when he was a hundred years old. Just as Isaac was indeed born during the time of the Old Testament, yet by his very birth he designates the heirs of the New Testament, so the circumcision by which he was consecrated is indeed a sacrament of the Old Testament, but it was foreshadowed in the figure of the grace of the New Testament, by which the world was to be cleansed in Christ from all filth of sin, death, and mortality. For at that time circumcision freed the faithful from the bond of original transgression; yet it was given as a type of the higher grace, by which the entire kingdom of sin and death was to be destroyed by the passion and resurrection of the Lord; in the image of which we also are absolved from all sins in baptism, and on the last day, renewed from all corruption and mortality of flesh and soul, we will reach eternal life; where, as the Lord said, the sons of the resurrection neither marry nor are given in marriage, but are like angels in heaven; for they can no longer die (Mark 12:25). And because the Lord rose again from the dead on the eighth day, that is, after the Sabbath, we also hope to rise again in the eighth age; for there are six ages of this world, the seventh is the Sabbath rest of souls in another life, the eighth itself is of our resurrection and the universal judgement; therefore, it was rightly commanded that circumcision be performed on the eighth day. But the fact that Abraham was a hundred years old when Isaac was born and his son of the promise was circumcised, most fittingly corresponds to the perfection of that same promise. For since a hundred is a perfect number, which is especially evidenced by the fact that it passes from the left hand to the right; therefore, it mystically pertains to heavenly and perpetual goods, Isaac is rightly born into this, who by his birth born miraculously to aged parents, would designate the heirs not of a temporal and lowly kingdom, but of the eternal kingdom in the heavens, in which indeed as a sacrament, Noah's ark was built over a hundred years, and Abraham dwelt a hundred years in the land of promise, and Isaac, sowing in Gerar, found a hundredfold in that very year, and the court of the tabernacle is a hundred cubits long; and in the parable of the gospel seed, the good ground produced a hundredfold fruit, and the…