Genesis 22:16

This is Isaac the co-heir of the promises and the blessings of God given to Abraham his father—who was a type of the sacrifice of the Lord Christ, since for three days he travelled on to death, and afterwards returned alive—who on his own shoulders carried the wood for his own sacrifice, as also the Lord Christ carried his own cross on his shoulder—who died in intention and was given his life by God; he in exchange for whom a ram was slain, and whose father heard these words from God: Because thou hast not spared the son whom thou lovest, so in like manner it has been said with reference to Christ the son of God: Who spared not his own son but has given him up for us all; although the flesh alone is that which has been given for the life of the world, since it is impossible for deity to die; but since the flesh has thus been given, scripture saith that his own son hath been given, because the flesh is a substitute for and a counterpart of the son, after the example of the blessed Isaac. For thus saith the Lord: Abraham rejoiced to see my day, and he saw it, and was glad.

In the former there was only speech, here an oath is interposed, which the holy Apostle, writing to the Hebrews, interprets, saying in this manner: "God," he says, "wishing to show to the heirs of the promise the immutability of his purpose, interposed an oath." And again: "men," he says, "swear by one greater than themselves." "But God, since he had no one greater by whom he might swear," "by myself I swear, said the Lord." Not that any necessity of an oath pressed upon God (for who would exact an oath from him?), but, as the Apostle Paul interpreted it, that by this he might "designate the immutability of his purpose to his worshippers." So too elsewhere it is said through the prophet: "the Lord has sworn and he will not repent: you are a priest forever according to the order of Melchizedek." Then, further, in the first promise it does not have set down a cause why the promise was given, only that he led him out and "showed him," it says, "the stars of heaven, and said: so shall your seed be." But now it adds the cause on account of which by an oath it confirms that the promise will be firm. For it says: "because you have done this word and have not spared your son." It shows, therefore, that the promise is firm on account of the offering or passion of the son, evidently indicating that on account of the passion of Christ the promise remains firm for the people from the nations, "which is of the faith of Abraham." And is it only in this that the second things are firmer than the first? In many things of this kind you will find mysteries adumbrated. The first tablets of the law Moses broke in the letter and cast away; the second law he received in the spirit, and the second are firmer than the first. Again, this same man, when he had comprised the whole law in four books, writes Deuteronomy, which is called the second law. Ishmael is first, Isaac second, and in the second a like form of preferment is kept. This too you will find adumbrated in like manner in Esau and Jacob, in Ephraim and Manasseh, and in a thousand others.