Some notes about death from Augustine's City of God, XIII.1-6:
Human beings experience death because of sin. Death would not have interposed if they had “continued in perfect obedience” (XIII.1).
There’s a distinction between the death of the soul (“when God abandons it”) and the death of the body (“when the soul departs”). Thus “the death of the whole man ... comes when the soul, abandoned by God, leaves the body.” this is “followed by ... ‘the second death’,” when both body and soul are destroyed in Gehenna. This “second death” entails not the separation of body and soul, but their being reunited. The body “remains possessed of soul and felling, and endures torment in this condition.” though this is not “death” in the sense of “separation,” it is death in the sense of pain, which is an evil. Consequently, because he feels “the anguish of punishment ... his condition is rightly called death rather than life.” (XIII.2)
The “first death” is that in which there is “a separation of natures which cohere together, either God and the soul, or the soul and the body.” Thus “the first death ... is good for the good” but “bad for the bad,” which none of the good experience the second death at all (XIII.2).
The key question now arises: “How can death, which separates body and soul, really be a good thing for the good” since “death is itself the penalty for sin?” Indeed, if death is the penalty for sin, if shouldn’t happen to “the good” at all, for “why should there be any punishment where there were no sins?” (XIII.3).
It’s true that Adam and Eve would not have experienced any kind of death if they had not sinned. Yet after their sin, all their descendants were subject to the same punishment. Indeed, “the condemnation” to which Adam and Eve were subject “changed human nature for the worse; so that what first happened as a matter of punishment” continued thereafter as “something natural.” (XIII.3).
This inherited subjection to death arises because in the production of human offspring the children share the nature of the parent. This stands in contrast to the production of the first man, where Adam was made from dust, a different substance altogether. This appears to be connected with Augustine’s traducianism (if the glimmer of anachronism may be excused here), whereby “the whole human race was in the first man, and it was to pass from him through the woman into his progeny.” A concise summary: “it was not man as first made, but what man became after his sin and punishment, that was thus begotten.” Again, “Human nature in [Adam] was vitiated and altered ... and he produced offspring in the same condition” (XIII.3).
It would be possible to read this in a crudely substantialist manner, whereby “human nature” is reified and conceived of as a kind of “stuff” passed on from parent to child. Indeed, it’s possible that this is what Augustine had in mind. But we don’t need to establish exactly what Augustine meant at this point (and a good job too, since the enterprise would be daunting indeed). We need merely note that Augustine’s notion of a vitiated human nature does not itself require such a crude substantialist reification of human nature. Augustine’s conception (no pun intended) makes complete sense within a relational ontology, within which “human nature” is constituted by virtue of the relationships sustained between the human subject and other people, animals, and of course God himself.
Augustine returns at this point to “the question why those whose guilt is removed through grace should suffer the death which is the penalty for sin” (XIII.4). To his first answer (what was penal in Adam became natural in his offspring), he now adds a second, already discussed in his book On the Baptism of Infants. There he explains “that the separation of soul from body remains, although its connection with guilt is removed, because if the immortality of the body followed immediately upon the sacrament of regeneration, faith itself would be weakened, since faith is only faith when what is not yet seen in reality is awaited in hope.” (XIII.4).
Two clarifications may be helpful here. First, this answer clearly builds on the previous one. The previous answer explains on “natural” grounds why death should persist in Adam’s redeemed offspring, but it does not by itself explain what good purpose is served by the (physical) death of the faithful. To put it another way, the first answer explains how it is that those cleansed of sin should suffer sin’s penalty; this second answer explains why.
Second, Augustine’s answer does not depend on accepting his doctrine of baptism (the “sacrament of regeneration,” as he calls it). If you prefer, simply replace this phrase in the above quotation with “the new birth” or “regeneration” or whatever else fits your theology. Augustine’s central point remains: faith itself requires that the thing believed in should not (yet) be seen.
Augustine now suggests a third reason for the persistence of death in the redeemed. It allowed even “the fear of death” to be conquered by the martyrs of the early church. If the redeemed were spared even the experience of physical death, then death itself would in one sense have been conquered, and yet the spectre of death would have remained as an enemy to be feared. But by allowing physical death – the death of the body – to hold sway over the redeemed, God allows an opportunity for the fear of death to be overcome by those threatened with execution for their faith in Christ. Not even the thought of death, the fear of death, need now hold any sway over the redeemed. And this arises not from a glib, naive profession of “life after death,” but precisely from confronting death itself. Thus Christ conquered death, and faith conquers the fear of death.
“So by the ineffable mercy of God even the penalty of man’s offence is turned into an instrument of virtue.” (XIII.4). Where previously it was said to Adam, “If you sin, you will die,” now it is said to the martyrs, “If you die, you avoid sin” (not here quoting Augustine). Or, in Augustine’s phrase, “Then death was purchased by sinning; now righteousness is fulfilled by dying.” Yet “It is not that death has turned into a good thing,” but rather that God has ordained that faith unto death shall “become the means whereby men pass into life.” (XIII.4).
Thus death itself remains bad – a terrible, aberrant intrusion into God’s good creation – while at the same time God paradoxically turns it back on itself so that it should by faith become the path to good. It’s rather like “the law” in Romans 7 (XIII.5). In this respect (among others) it is possible to “die a good death” (XIII.5).
Indeed, death itself, “the violent sundering of two elements, which are conjoined and interwoven in a living being, is bound to be a harsh and unnatural experience.” Thus death itself remains not only bad, but also painful. Yet, we see again, its character is transformed. It retains the term “punishment,” for it is “the penalty of all who are born, yet it becomes the glory of all who are reborn.” (XIII.6).