54. Gn. litt. 6.25; Hill, Genesis, 321; CSEL 28.197.
56 Made to be Godly manifest Adam's divine similitude: (1) immortality, (2) impeccability, (3) perfect tranquility, and (4) bodily integrity.
First is Adam's immortality. Adam was not created to die but was given the gift of everlasting life, provided he remained obedient unto God. Here Augustine makes his well-known distinction between the ability not to die [posse non mori] and the perfect state of not being able to die [non posse mori]. We read that in the beginning the human condition was one of both mortality, in that death was a possibility, and immortality, in that it was certainly God's intended state. Augustine explains:
Before sin, in fact, the body could have been said to be both mortal for one reason and immortal for another; mortal, that is, because it was able to die, and immortal because it was not able not to die. It is one thing, after all, not to be able to die, like natures which God created immortal, while it is quite another to be able not to die; and this is the way the first man was created immortal, something to be granted him by means of the tree of life, not by his natural constitution. From this tree of life he was cut off when he had sinned, so that he could die, while if he had not sinned he would have been able not to die.54
Augustine first describes the human body as teetering between two extremes: it is created between God and nothing, as well as between immortality and nothingness. The destiny of the human body will be decided by which way its soul turns. He next distinguishes between two different states of immortality: that which some natures have per se and that which some enjoy through grace only (and therefore contingent upon something other than their own nature). The immortality for which Adam was created was not something absolute and necessary but something dependent upon his obedience to God. In his disobedience, Adam was severed [separatus est] from the source of immortality; death thus became the result of human rebellion.
54. Gn. litt. 6.25; Hill, Genesis, 321; CSEL 28.197.