The book is available online at CCEL, in the same translation of the book in my hand. It can also be downloaded from the online page in PDF format, but then you need to be registered (for free) as a member.
Anyway, Augustine, concerning the reason for the harsh punishment on Adam and Eve for their sin, says the following:
"Therefore, because the sin was a despising of the authority of God,—who had created man; who had made him in His own image; who had set him above the other animals; who had placed him in Paradise; who had enriched him with abundance of every kind and of safety; who had laid upon him neither many, nor great, nor difficult commandments, but, in order to make a wholesome obedience easy to him, had given him a single very brief and very light precept by which He reminded that creature whose service was to be free that He was Lord,—it was just that condemnation followed, and condemnation such that man, who by keeping the commandments should have been spiritual even in his flesh, became fleshly even in his spirit; and as in his pride he had sought to be his own satisfaction, God in His justice abandoned him to himself, not to live in the absolute independence he affected, but instead of the liberty he desired, to live dissatisfied with himself in a hard and miserable bondage to him to whom by sinning he had yielded himself, doomed in spite of himself to die in body as he had willingly become dead in spirit, condemned even to eternal death (had not the grace of God delivered him) because he had forsaken eternal life. Whoever thinks such punishment either excessive or unjust shows his inability to measure the great iniquity of sinning where sin might so easily have been avoided." (Book XIV, 15)
I am not in complete agreement on this.
I can see why Augustine would say that the punishment was so severe. To Augustine, the severity of the punishment is the inverse of the ease with which God's command could have been kept, i.e. Because it should have been so incredibly easy for our first parents to refuse to eat from "the tree of the knowledge of good and evil" (Gen 2:17), God decided to punish them very harshly.
Augustine's idea doesn't necessarily follow, since I am not so sure that the Biblical data points this way. James in his NT book says the following:
"(10) For whoever keeps the whole law but fails in one point has become accountable for all of it. (11) For he who said, 'Do not commit adultery,' also said, 'Do not murder.' If you do not commit adultery but do murder, you have become a transgressor of the law." (Js 2:10-11)
According to James, it is not the level of difficulty in keeping the law that is important, but the One who gave the law. It is not even the individual commandment within the law, but the One who gave that commandment. James highlights the fact that the One who said 'Do not commit adultery,' is the same One who said, 'Do not murder.' So, it is the importance of the Lawgiver that should draw our attention, not the individual commandments of the law.
Granted, the first law only contained one commandment: "Don't eat from THAT tree!" However, the severity of the punishment on mankind, not just on our protoparents, has nothing to do with the ease of keeping the law, but with the Lawgiver Himself.
So, this gets us somewhere.
In October 2005, in a blog post I entitled "What is the Gospel? What is the foundation to the Gospel?," I wrote the following:
"God is an infinitely holy God, and His justice therefore demands infinite vindication before the righteousness of God. Without an infinite payment (which only Christ could fulfill), only an eternal payment by sinners could even begin to vindicate the holiness of God."
When we sin, no matter how "small" the sin, we are sinning against a holy God. Since God is infinite, we have to agree that His holiness is also infinite. Therefore, the punishment for sin against infinite holiness must be commensurate with the holiness that was infringed upon. Remember, sin is not merely the breaking of the law. It is disobedience against a infinitely holy God! Propitiation for sin against infinite holiness cannot be a simple slap on the wrist! That would be a gross misunderstanding of who God is!
So, the reason our protoparents, and through them us, were punished so severely, is that the payment for their sin had to be an infinite payment to satisfy the wrath of an infinitely holy God.
Thank God for sending Jesus, that He could be "the propitiation for our sins!" (1 Jn 4:10) Only an infinite sacrifice could satisfy infinite holiness!
Where can you buy The City of God?
Amazon US - $12.44
Amazon UK - £9.23
Kalahari.net RSA - R199.71
Christianbook.com - $12.99