4 For, Brethren, the soul is said to be immortal, and immortal it is according to a certain manner of its own: for it is a kind of life which is able to give life to the body by its presence. For by the soul does the body live. This life cannot die, and therefore is the soul immortal. Why then said I according to a certain manner of its own? Hear why. Because there is a true immortality, an immortality which is an entire unchangeableness; of which the Apostle says, speaking of God, “Who only has immortality, dwelling in that light which no man may approach unto, whom no man has seen, nor can see, to whom be honour and glory for ever and ever.
Amen.” If then God only has immortality, the soul must needs be mortal. See then why it was that I said that the soul is immortal after a certain manner of its own. For in fact it may also die. Understand this, Beloved, and there will remain no difficulty. I venture to say then that the soul can die, can be slain also. Yet it is undoubtedly immortal. See, I venture to say, it is at once immortal, and it may be slain; and therefore I said that there is a kind of immortality, an entire unchangeableness, that is, which God Only has, of whom it is said, “Who Only has immortality;” for if the soul cannot be slain, how did the Lord Himself say, when He would make us fear, “Fear Him who has power to slay both body and soul in hell”?
Source: Sermons on Selected Lessons of the New Testament (New Advent)