Such are the notions which men put forward. But the impiety of their foolish talk is plainly declared by the divine teaching of the Christian faith. From it we know that, because there is Mind behind the universe, it did not originate itself; because God is infinite, not finite, it was not made from pre-existent matter, but out of nothing and out of non-existence absolute and utter God brought it into being through the Word. He says as much in Genesis: "In the beginning God created the heavens and the earth"; and again through that most helpful book The Shepherd, "Believe thou first and foremost that there is One God Who created and arranged all things and brought them out of non-existence into being." Paul also indicates the same thing when he says, "By faith we understand that the worlds were framed by the Word of God, so that the things which we see now did not come into being out of things which had previously appeared." For God is good—or rather, of all goodness He is Fountainhead, and it is impossible for one who is good to be mean or grudging about anything. Grudging existence to none therefore, He made all things out of nothing through His own Word, our Lord Jesus Christ and of all these His earthly creatures He reserved especial mercy for the race of men. Upon them, therefore, upon men who, as animals, were essentially impermanent, He bestowed a grace which other creatures lacked—namely the impress of His own Image, a share in the reasonable being of the very Word Himself, so that, reflecting Him and themselves becoming reasonable and expressing the Mind of God even as He does, though in limited degree they might continue for ever in the blessed and only true life of the saints in paradise. But since the will of man could turn either way, God secured this grace that He had given by making it conditional from the first upon two things—namely, a law and a place. He set them in His own paradise, and laid upon them a single prohibition. If they guarded the grace and retained the loveliness of their original innocence, then the life of paradise should be theirs, without sorrow, pain or care, and after it the assurance of immortality in heaven. But if they went astray and became vile, throwing away their birthright of beauty, then they would come under the natural law of death and live no longer in paradise, but, dying outside of it, continue in death and in corruption. This is what Holy Scripture tells us, proclaiming the command of God, "Of every tree that is in the garden thou shalt surely eat, but of the tree of the knowledge of good and evil ye shall not eat, but in the day that ye do eat, ye shall surely die." "Ye shall surely die"—not just die only, but remain in the state of death and of corruption.
Part 3 of Chapter 1 of "On the Incarnation"