From City of God, 16.26 (ca. 426)
Here there are more distinct promises about the calling of the nations in Isaac, that is, in the son of the promise, by which grace is signified, and not nature; for the son is promised from an old man and a barren old woman. For although God effects even the natural course of procreation, yet where the agency of God is manifest, through the decay or failure of nature, grace is more plainly discerned.
And because this was to be brought about, not by generation, but by regeneration, circumcision was enjoined now, when a son was promised of Sarah. And by ordering all, not only sons, but also home-born and purchased servants to be circumcised, he testifies that this grace pertains to all. For what else does circumcision signify than a nature renewed on the putting off of the old? And what else does the eighth day mean than Christ, who rose again when the week was completed, that is, after the Sabbath? The very names of the parents are changed: all things proclaim newness, and the new covenant is shadowed forth in the old. For what does the term old covenant imply but the concealing of the new? And what does the term new covenant imply but the revealing of the old? The laughter of Abraham is the exultation of one who rejoices, not the scornful laughter of one who mistrusts. And those words of his in his heart, “Shall a son be born to me that am an hundred years old? and shall Sarah, that is ninety years old, bear?” are not the words of doubt, but of wonder.
And when it is said, “And I will give to thee, and to thy seed after thee, the land in which thou art a stranger, all the land of Canaan, for an everlasting possession,” if it troubles any one whether this is to be held as fulfilled, or whether its fulfilment may still be looked for, since no kind of earthly possession can be everlasting for any nation whatever, let him know that the word translated everlasting, by our writers is what the Greeks term αἰώνιον, which is derived from αἰὼν, the Greek for saeculum, an age. But the Latins have not ventured to translate this by secular, lest they should change the meaning into something widely different. For many things are called secular which so happen in this world as to pass away even in a short time; but what is termed αἰωνιον either has no end, or lasts to the very end of this world.
St. Augustine (354-430) was a theologian and philosopher who served as Bishop of Hippo Regius in North Africa. He was a voluminous author, whose writings about God’s grace, the Sacraments, and the Church have been profoundly influential in the development of Western Christianity The City of God, his magnum opus, considers the relationship between human history and Divine Providence. His feast day is August 26.