Objection 1. It would seem that in the state of innocence man was not immortal. For the term "mortal" belongs to the definition of man. But if you take away the definition, you take away the thing defined. Therefore as long as man was man he could not be immortal.
Objection 2. Further, corruptible and incorruptible are generically distinct, as the Philosopher says (Metaph. x, Did. ix, 10). But there can be no passing from one genus to another. Therefore if the first man was incorruptible, man could not be corruptible in the present state.
Objection 3. Further, if man were immortal in the state of innocence, this would have been due either to nature or to grace. Not to nature, for since nature does not change within the same species, he would also have been immortal now. Likewise neither would this be owing to grace; for the first man recovered grace by repentance, according to Wisdom 10:2: "He brought him out of his sins." Hence he would have regained his immortality; which is clearly not the case. Therefore man was not immortal in the state of innocence.
Objection 4. Further, immortality is promised to man as a reward, according to Apocalypse 21:4: "Death shall be no more." But man was not created in the state of reward, but that he might deserve the reward. Therefore man was not immortal in the state of innocence.
On the contrary, It is written (Romans 5:12): "By sin death came into the world." Therefore man was immortal before sin.
I answer that, A thing may be incorruptible in three ways.
First, on the part of matter--that is to say, either because it possesses no matter, like an angel; or because it possesses matter that is in potentiality to one form only, like the heavenly bodies. Such things as these are incorruptible by their very nature.
Secondly, a thing is incorruptible in its form, inasmuch as being by nature corruptible, yet it has an inherent disposition which preserves it wholly from corruption; and this is called incorruptibility of glory; because as Augustine says (Ep. ad Dioscor.): "God made man's soul of such a powerful nature, that from its fulness of beatitude, there redounds to the body a fulness of health, with the vigor of incorruption."
Thirdly, a thing may be incorruptible on the part of its efficient cause; in this sense man was incorruptible and immortal in the state of innocence. For, as Augustine says (QQ. Vet. et Nov. Test. qu. 19 [Work of an anonymous author, among the supposititious works of St. Augustine): "God made man immortal as long as he did not sin; so that he might achieve for himself life or death." For man's body was indissoluble not by reason of any intrinsic vigor of immortality, but by reason of a supernatural force given by God to the soul, whereby it was enabled to preserve the body from all corruption so long as it remained itself subject to God. This entirely agrees with reason; for since the rational soul surpasses the capacity of corporeal matter, as above explained (76, 1), it was most properly endowed at the beginning with the power of preserving the body in a manner surpassing the capacity of corporeal matter.
Reply to Objection 1 and 2 These objections are founded on natural incorruptibility and immortality.
Reply to Objection 3. This power of preserving the body was not natural to the soul, but was the gift of grace. And though man recovered grace as regards remission of guilt and the merit of glory; yet he did not recover immortality, the loss of which was an effect of sin; for this was reserved for Christ to accomplish, by Whom the defect of nature was to be restored into something better, as we shall explain further on (III, 14, 4, ad 1).
Reply to Objection 4. The promised reward of the immortality of glory differs from the immortality which was bestowed on man in the state of innocence.
Source: Summa Theologica (New Advent)