1 He had indeed been already accustomed to lie against God, for the purpose of leading men astray. For at the beginning, when God had given to man a variety of things for food, while He commanded him not to eat of one tree only, as the Scripture tells us that God said to Adam: “From every tree which is in the garden you shall eat food; but from the tree of knowledge of good and evil, from this you shall not eat: for in the day that you shall eat of it, you shall die by death;” he then, lying against the Lord, tempted man, as the Scripture says that the serpent said to the woman: “Has God indeed said this, You shall not eat from every tree of the garden?” And when she had exposed the falsehood, and simply related the command, as He had said, “From every tree of the garden we shall eat; but of the fruit of the tree which is in the midst of the garden, God has said, You shall not eat of it, neither shall you touch it, lest you die:” when he had [thus] learned from the woman the command of God, having brought his cunning into play, he finally deceived her by a falsehood, saying, “You shall not die by death; for God knew that in the day you shall eat of it your eyes shall be opened, and you shall be as gods, knowing good and evil.” In the first place, then, in the garden of God he disputed about God, as if God was not there, for he was ignorant of the greatness of God; and then, in the next place, after he had learned from the woman that God had said that they should die if they tasted the aforesaid tree, opening his mouth, he uttered the third falsehood, “You shall not die by death.” But that God was true, and the serpent a liar, was proved by the result, death having passed upon them who had eaten. For along with the fruit they did also fall under the power of death, because they ate in disobedience; and disobedience to God entails death. Wherefore, as they became forfeit to death, from that [moment] they were handed over to it.
2. Thus, then, in the day that they ate, in the same did they die, and became death's debtors, since it was one day of the creation. For it is said, “There was made in the evening, and there was made in the morning, one day.” Now in this same day that they ate, in that also did they die. But according to the cycle and progress of the days, after which one is termed first, another second, and another third, if anybody seeks diligently to learn upon what day out of the seven it was that Adam died, he will find it by examining the dispensation of the Lord. For by summing up in Himself the whole human race from the beginning to the end, He has also summed up its death. From this it is clear that the Lord suffered death, in obedience to His Father, upon that day on which Adam died while he disobeyed God. Now he died on the same day in which he ate. For God said, “In that day on which you shall eat of it, you shall die by death.” The Lord, therefore, recapitulating in Himself this day, underwent His sufferings upon the day preceding the Sabbath, that is, the sixth day of the creation, on which day man was created; thus granting him a second creation by means of His passion, which is that [creation] out of death. And there are some, again, who relegate the death of Adam to the thousandth year; for since “a day of the Lord is as a thousand years,” he did not overstep the thousand years, but died within them, thus bearing out the sentence of his sin. Whether, therefore, with respect to disobedience, which is death; whether [we consider] that, on account of that, they were delivered over to death, and made debtors to it; whether with respect to [the fact that on] one and the same day on which they ate they also died (for it is one day of the creation); whether [we regard this point], that, with respect to this cycle of days, they died on the day in which they did also eat, that is, the day of the preparation, which is termed “the pure supper,” that is, the sixth day of the feast, which the Lord also exhibited when He suffered on that day; or whether [we reflect] that he (Adam) did not overstep the thousand years, but died within their limit—it follows that, in regard to all these significations, God is indeed true. For they died who tasted of the tree; and the serpent is proved a liar and a murderer, as the Lord said of him: “For he is a murderer from the beginning, and the truth is not in him.”
Source: Adversus haereses (New Advent)