"By nature, of course, man is mortal, since he was made from nothing; but he bears also the Likeness of Him Who is, and if he preserves that Likeness through constant contemplation, then his nature is deprived of its power and he remains incorrupt."
Athanathius believed that if something is created out of nothing, it has the potential to become nothing again. Because God saw that if He created human beings, they would be created out of nothing, and therefore could not live forever, He gave them a further gift by making them in His image and letting them share in the power of the Word.
"Among these things, of all things upon earth he had mercy upon the human race, and seeing that by the principle of its own coming into being it would not be able to endure eternally, he granted them a further gift, creating human beings not simply like all the irrational animals upon the earth but making them according to his own image, giving them a share of the power of his own Word, [...]" - On the Incarnation ch 3
Athanatius does not mean that humans were created corrupted or tending towards death, they were created in God's image and therefore in communion with Him. When he says humanity sinned and was "returned to the natural state" he means the state of things that were created out of nothing apart from the image of God/participation in the Word. We can see this here:
"they received the previously threatened condemnation of death, and thereafter no longer remained as they had been created, but were corrupted as they had contrived; and, seizing them, death reigned. For the transgression of the commandment returned them to their natural state, so that, just as they, not being, came to be, so also they might rightly endure in time the corruption to non-being" - On the Incarnation ch 3 (emphasis mine)
We can see clearly that when Athanathius talks about "natural state" he does not mean human nature as it was created, but human nature apart from the image of God. So humanity, as created, was not neutral, but good, and as long as they obeyed God's laws they would remain uncorrupted and attain immortality.
For Athanatius good is being as it comes from the eternal God, and evil is non-being. Therefore, when we sinned and turned to evil we lost eternal being.
"For if, having a nature that did not once exist, they were called into existence by the Word's advent and love for human beings, it followed that when human beings were bereft of the knowledge of God and had turned to things which exist not- evil is non-being, the good is being, since it has come into being from the existing God- then they were bereft also of eternal being." - On the Incarnation ch 3
Athanatius then takes this principle and goes further, saying that because of our great sin and the breaking of God's law the corruption of humanity is even greater than that of our "natural state".
"For God has not only created us from nothing, but also granted us by the grace of the Word to live a life according to God. But human beings, turning away from things eternal and by the counsel of the devil turning us towards things of corruption, were themselves the cause of corruption in death, being, as we already said, corruptible by nature but escaping their natural state by the grace of participation in the Word, had they remained good. Because of the Word present in them, even natural corruption did not come near them, just as Wisdom says, "God created the human being for incorruptibility and as an image of his own eternity; but by the envy of the devil, death entered into the world". When this happened, human beings died and corruption thenceforth prevailed against them, becoming even stronger than its natural power over the whole race, the more so as it had assumed the threat of Deity against them through the transgression of the commandment." - On the Incarnation ch 5
I think nature is being used in a very strange way here, it seems to be almost hypothetical, because I would have thought because humanity is created in the image of God that
is part of our nature.
Because Athanasius still says that humanity were created in God's image and therefore were created good, and that God revealed His law to Adam in the garden to show him that if he remained good he would inherit eternal life, but if he turned away he would die, it seems he rejects humanity being created as purely neutral, which is good. On top of that, although he talks a lot about being and non-being, he seems to avoid a lot of the problems with a "chain of being" view.
I still need to think more about this "natural corruption", and I don't know anything about EO views on Adam, so that's about as much as I can say on the matter. I think you can see where some of the thoughts here develop towards theosis, but there is some connection to Reformed thought here too.
