A very nice summary, with patristic citations, can be found at: Free Will and Death
Below are several lengthy citations from Sts. Athanasios (On the Incarnation of the Word), John Chrysostom (Homilies on Genesis and on Romans), and John of Damascus (An Exact Exposition of the Orthodox Faith).
St. Athanasius, On the Incarnation of the Word
For He brought them into His own garden, and gave them a law: so that, if they kept the grace and remained good, they might still keep the life in paradise without sorrow or pain or care besides having the promise of incorruption in heaven; but that if they transgressed and turned back, and became evil, they might know that they were incurring that corruption in death which was theirs by nature: no longer to live in paradise, but cast out of it from that time forth to die and to abide in death and in corruption. Now this is that of which Holy Writ also gives warning, saying in the Person of God: “Of every tree that is in the garden, eating thou shalt eat: but of the tree of the knowledge of good and evil, ye shall not eat of it, but on the day that ye eat, dying ye shall die.” But by “dying ye shall die,” what else could be meant than not dying merely, but also abiding ever in the corruption of death?(3.4-5)
Thus, then, God has made man, and willed that he should abide in incorruption; but men, having despised and rejected the contemplation of God, and devised and contrived evil for themselves (as was said in the former treatise), received the condemnation of death with which they had been threatened; and from thenceforth no longer remained as they were made, but were being corrupted according to their devices; and death had the mastery over them as king. For if, out of a former normal state of nonexistence, they were called into being by the Presence and loving-kindness of the Word, it followed naturally that when men were bereft of the knowledge of God and were turned back to what was not (for what is evil is not, but what is good is), they should, since they derive their being from God who IS, be everlastingly bereft even of being; in other words, that they should be disintegrated and abide in death and corruption. For man is by nature mortal, inasmuch as he is made out of what is not; but by reason of his likeness to Him that is (and if he still preserved this likeness by keeping Him in his knowledge) he would stay his natural corruption, and remain incorrupt; as Wisdom says: “The taking heed to His laws is the assurance of immortality;” but being incorrupt, he would live henceforth as God, to which I suppose the divine Scripture refers, when it says: “I have said ye are gods, and ye are all sons of the most Highest; but ye die like men, and fall as one of the princes.”For God has not only made us out of nothing; but He gave us freely, by the Grace of the Word, a life in correspondence with God. But men, having rejected things eternal, and, by counsel of the devil, turned to the things of corruption, became the cause of their own corruption in death, being, as I said before, by nature corruptible, but destined, by the grace following from partaking of the Word, to have escaped their natural state, had they remained good. For because of the Word dwelling with them, even their natural corruption did not come near them, as Wisdom also says: “God made man for incorruption, and as an image of His own eternity; but by envy of the devil death came into the world.” But when this was come to pass, men began to die, while corruption thence-forward prevailed against them, gaining even more than its natural power over the whole race, inasmuch as it had, owing to the transgression of the commandment, the threat of the Deity as a further advantage against them.(4.4-5.2)
What the saint is saying above is not that death and corruption are the per se intended state of man, but rather that man does not have in himself the principle of life. That is to say, we are not inherently immortal, but are only living and immortal insofar as we remain in communion with God. Only God is truly immortal and alive in se. For man all his life and immortality is contingent. It is thus "natural" to man to be alive and immortal in that this is God's intended will for man. But insofar as man rejects God's will, he cuts himself off from that life and immortality, and thus, not having life and immortality in himself "naturally" succumbs to death and corruption, which is "unnatural" in that it is not God's will for man.
For this purpose, then, the incorporeal and incorruptible and immaterial Word of God comes to our realm, howbeit he was not far from us before. For no part of Creation is left void of Him: He has filled all things everywhere, remaining present with His own Father. But He comes in condescension to shew loving-kindness upon us, and to visit us. And seeing the race of rational creatures in the way to perish, and death reigning over them by corruption; seeing, too, that the threat against transgression gave a firm hold to the corruption which was upon us, and that it was monstrous that before the law was fulfilled it should fall through: seeing, once more, the unseemliness of what was come to pass: that the things whereof He Himself was Artificer were passing away: seeing, further, the exceeding wickedness of men, and how by little and little they had increased it to an intolerable pitch against themselves: and seeing, lastly, how all men were under penalty of death: He took pity on our race, and had mercy on our infirmity, and condescended to our corruption, and, unable to bear that death should have the mastery—lest the creature should perish, and His Father’s handiwork in men be spent for nought—He takes