I was going to post a comment with some thoughts directly to the above thread but it is a closed forum: I can not join because I can agree to their statement of faith.
However here is an extract from the original post
Does anyone believe that Adam and Eve had conditional immortality prior to the fall? Meaning that they had the ability to die, but were sustained by the tree of life?
The author quotes Erickson
“I would suggest the concept of conditional immortality as the state of Adam before the fall. He was not inherently able to live forever, but he need not have died. Given the right conditions, he could have lived on forever: This may be the meaning of God’s words when he decided to expel Adam and Eve from Eden and from the presence of the tree of life: “and now, lest he put forth his hand and take also of the tree of life, and eat, and live forever” (3:22). The impression is given that Adam, even after the fall, could have lived forever if he had eaten the fruit of the tree of life. What happened at the time of his explusion from Eden was that man, who formerly could have lived forever or died, was now separated from those conditions which made eternal life possible, and thus it became inevitable that he die. Previously he *could* die; now he *would* die. This also means that Jesus was born with a body that was subject to death. He had to eat to live; had he failed to eat he would have starved to death.
We should note that there were other changes as a result of sin. In Eden man had a body which could become diseased; after the fall there were diseases for him to contract. The curse, involving the coming of death to mankind, also included a whole host of ills which would lead to death. Paul tells us that someday this set of conditions will be removed, and the whole creation delivered from this ‘bondage to decay’ (Rom 8:18-23).
To sum up: the potential of death was within the creation from the beginning. But the potential of eternal life was also there. Sin, in the case of Adam and each of us, means that death is no longer merely potential but actual.”
Erickson, “Christian Theology” p. 613
This would seem to be the physical counterpart to posse peccare, posse non peccare. The conditional aspect of immortality would have been removed had Adam fulfilled the covenant.
Both John Calvin and Millard Erickson spoke of pre-fall humanity as capable of not sinning (posse non peccare) before the fall, thus being conditionally immortal. The idea being that if Adam and Eve had not sinned they would have eventually eaten of the tree of life and thus become immortal. Then both Calvin and Erickson (in my opinion) ruin their arguments by going on to say that in spite of breaking the prohibition, they still have immortality!
You can find some more thoughts: The Tree of Life and Conditional Immortality