We now have to speak of "death", which applies to mankind but not to angels.
Of course, in a sense man is immortal. We understand that man, soul and body, will live forever once he is created. That we die in the physical meaning of the word is a temporary situation for redeemed and reprobate alike. Which means that we have to ask exactly what it means that man "dies." This really applies only to the physical body, since the soul lives forever and the body (temporarily) disintegrates when God withdraws His sustaining power from it.
And yet, we also have to speak of a "total death for man, a death of body and soul, namely, when a soul, abandoned by God, abandons the body. In this case, the soul has no life from God and the body no life from the soul. The consequence of such total death is the second death..."
This death culminates when "soul and body are reunited, never to be separated again." But how can we speak of a body as "dead" if the soul is in torment but the body has not yet been resurrected? Well, that is a trickier proposition, but really we can talk that way because from the moment a person dies separated from God by sin, everything about that person's existence (both just as a soul and as a resurrected soul and body) is focused on pain. And we really can't talk about something as "life" if the animating force is nothing but suffering.
What's more, physical death can be a good thing: "It can be said of the first death, the death of the body, that it is good for saints and bad for sinners, but of the second that it is certainly good for no one and non-existent for the saints."
It is dangerous to say that death is "good for the saints", since death is clearly a punishment for sin. How can we say that a punishment is good? "Besides,if it could not have happened except to sinners, it should not even happen to the saints, let alone be good for them. For, why should those who deserve no punishment be punished?"
Augustine answers this by noting that had Adam and Eve not sinned, they would not have died. But their sin was so great that it echoes down through all of their progeny, along with their guilt. Which isn't to say that we ourselves aren't individually guilty as well--"No one was to be born of them who was less a sinner than they were." So great was Adam's rebellion "what was originally a penal condition for the first parents who sinned became a natural consequence in all their descendants."
This is because of the nature of the connection between human beings. As having sprung from the same parents, we share with them the same nature. (This also explains why Adam and Eve in their unfallen state could have different natures from the rest of us--they were made of dust, while we were made of them.) This is not to say that Adam was reset to childhood when he sinned, but that his nature was corrupted so much that "it suffers from the recalcitrance of a rebellious concupiscence and is bound by the law of death." So too with all of Adam's children, who share his nature. As Bishop Ryle said,
We can acknowledge that man has all the marks of a majestic temple about him - a temple in which God once dwelt, but a temple which is now in utter ruins - a temple in which a shattered window here, and a doorway there, and a column there, still give some faint idea of the magnificence of the original design, but a temple which from end to end has lost its glory and fallen from its high estate. And we say that nothing solves the complicated problem of man’s condition but the doctrine of original or birth-sin and the crushing effects of the fall.Even infants if they are in heaven (which Augustine does not here expand upon) are there not because they are sinless or innocent, but "through the grace of the Mediator Christ." The atoning work of Christ on the cross alone is the fount of salvation from our own rebellious natures. (I won't link to it, what with being too lazy to look it up, but Augustine has a wonderful passage in the Confessions where he talks about observing babies and what sinners they are, even before they can speak or communicate beyond a scream.)
But if we have grace, why do we even have to face physical death--the punishment for sin? Augustine gives a couple of reasons, first that (as he expands on in his treatise on baptism) salvation is by faith alone, and if we were to immediately receive the fulfillment of our hope, we could hardly call what saved us "faith"--it would be sight instead.
Likewise, Augustine (again, speaking as a pastor) invites us to wonder at the majesty of God who takes something as awful as the punishment of death and turns it to something good. Specifically, he turns it to something good for us and for His glory: "Who would not run to join the infants about the be baptized, if the main purpose of Christ's grace were to save us from bodily death?" Everyone would be on board with that, and we would have no way to distinguish the true believers from those who just wish to avoid physical death.*
Finally, and most importantly, what was once a mark of our shame and our sin and our rebellion against God has become a means of sanctification and a sign of obedience:
But now, by a greater and more wonderful grace of the Saviour, the punishment of sin serves the purposes of sanctity. In the beginning, the first man was warned: 'if you sin, you shall die'; now, the martyr is admonished; 'die that you may not sin.'.... Thus, by the ineffable mercy of God, the penalty of sin is transformed into the panoply of virtue and the punishment of the sinner into the testing of the saint... The saints choose to suffer for their faith what the first sinners had no choice but to suffer for their infidelity. The sinners would not have died unless they sinned; the saints will sin unless they die.Not that we rush to embrace death like mindless fanatics! "This does not mean that death, which before was an evil, has now become something good." Rather, we understand that "God has rewarded faith with so much grace that death, which seems to be the enemy of life, becomes an ally that helps man enter into life."
The grace of God is so overwhelming that even death, the "last enemy to be destroyed", temporarily serves God as an ally in the salvation of His people.
*[And here we may note that Augustine's false view of baptism--and one quite at odds with his view of grace and salvation, though admittedly not at odds with his view of ecclesiology--trips him up a bit here. A more Biblical view of baptism understands that it does not regenerate, and so having to sort through who of the baptized are "true believers" and who are not has nothing to do with one's being dipped in water and everything to do with faith. Not that baptism is unimportant--let us never slight the ordinance commanded by our Lord!--but that we need rather to examine the faith of the person in the Gospel and its expression in love as the marks of salvation.]