9 I have, do you say, another method of showing; for so well instructed is my son, that he hears without my speaking, but I show him by a nod what to do. Lo, show him by a nod what you will, yet certainly the mind holds within itself that which it would show. By what do you give this nod? With the body—namely, with the lips, the look, the brows, the eyes, the hands. All these are not what your mind is: these, too, are media; there was something understood by these signs which are not what your mind is, not what the mind of your son is; but all this which you do by the body is beneath your mind, and beneath the mind of your son: nor can your son know your mind, unless you give him signs by the body.
What, then, do I say? This is not the case there; there all is simplicity. The Father shows to the Son what He is doing, and by showing begets the Son. I see what I have said; but because I see also to whom I have said it, may such understanding be some time or other formed in you as to grasp it. If you are not able now to comprehend what God is, comprehend at least what God is not: you will have made much progress, if you think of God as being not something other than He is. God is not a body, not the earth, not the heaven, not the moon, or sun, or stars— not these corporeal things.
For if not heavenly things, how much less is He earthly things! Put all body out of the question. Further, hear another thing: God is not a mutable spirit. For I confess,— and it must be confessed, for it is the Gospel that speaks it—“God is a Spirit.” But pass beyond all mutable spirit, beyond all spirit that now knows, now knows not; that now remembers, now forgets; that wills what before it willed not, that wills not what before it willed; either that suffers these mutabilities now or may suffer them: pass beyond all these.
Thou findest not any mutability in God; nor anything that may have been one way before, and is otherwise now. For where you find alternation, there a kind of death has taken place: since, for a thing not to be what it was, is a death. The soul is said to be immortal; so indeed it is, because it ever lives, and there is in it a certain continuous life, but yet a mutable life. According to the mutability of this life, it may be said to be mortal; because if it lived wisely, and then becomes foolish, it dies for the worse; if it lived foolishly, and becomes wise, it dies for the better.
For the Scripture teaches us that there is a death for the worse, and that there is a death for the better. In any case, they had died for the worse, of whom it said, “Let the dead bury their dead;” and, “Awake, you that sleepest, and arise from the dead, and Christ shall give you light;” and from this passage before us, “When the dead shall hear, and they that hear shall live.” For the worse they had died; therefore do they come to life again. By coming to life they die for the better, because by coming to life again they will not be what they were; but for that to be, which was not, is death.
But perhaps it is not called death if it is for the better? The apostle has called that death: “But if you be dead with Christ from the elements of this world, why do ye judge concerning this world as if you were still living?” And again, “For you are dead, and your life is hid with Christ in God.” He wishes us to die that we may live, because we have lived to die. Whatever therefore dies, both from better to worse, and from worse to better, is not God; because neither can supreme goodness proceed to better, nor true eternity to worse.
For true eternity is, where is nothing of time. But was there now this, now that? Immediately time is admitted, it is not eternal. For that you may know that God is not thus, as the soul is—certainly the soul is immortal—what, however, says the apostle of God, “Who alone has immortality,” unless that he openly says this, He alone has unchangeableness, because He alone has true eternity? Therefore no mutability is there.
Source: Tractates on the Gospel of John (New Advent)