Philosophy of St. Thomas - Immortality of the Body

What Adam lost at the Fall was not an immortality of the body that he possessed by nature; rather, it was an immortality of the body that he possessed by the grace of God.

St. Thomas says this as part of a discussion of the fact that our bodies are corruptible by nature in ST I Q76.

Perhaps someone might attempt to answer this by saying that before sin the human body was incorruptible. This answer does not seem sufficient; because before sin the human body was immortal not by nature, but by a gift of Divine grace; otherwise its immortality would not be forfeited through sin, as neither was the immortality of the devil (ibid., A5 ad 1).

Satan does not lose his immortality by his sin because he is immortal by his nature. We do not possess immortality by nature, or else we could not lose it either (because we would cease to have a human nature if our nature lost one of its essential characteristics). Rather, by Adam's sin his fellowship with God was broken, and he lost the grace by which he was preserved incorruptible (or by which he would have been incorruptible).