8 I see I must speak more plainly. And, not to detain you long, let me treat only of this point today. When I have finished what, with God's help, I wish to say, my discourse shall close. I have said this, then, to enlist your attention. Every soul, as being a thing, is mutable; and although a great creature, yet a creature; though superior to the body, yet made. Every soul, then, since it is changeable— that is, sometimes believes, sometimes disbelieves; at one time wishes, at another time refuses; at one time is adulterous, at another chaste; now good, and again wicked—is changeable.
But God is that which is, and so has retained as His own peculiar name, “I am who am.” Such also is the Son, when He says, “If you believe not that I am;” and thereto pertains also, “Who are you? The Beginning”. God therefore is unchangeable, the soul changeable. When the soul receives from God the elements of its goodness it becomes good by participation, just as by participation your eye sees. For it sees not when the light is withdrawn, while so long as it shares in the light it sees.
Since then by participation the soul is made good, if it changes and becomes bad, the goodness remains that made it good. For there is a goodness of which it partook when good; and when it has turned to evil, that goodness continues entire. If the soul fall away and become evil, there is no lessening of goodness; if it return and become good, that goodness is not enlarged. Your eye participates in this light, and you see. Is it shut? Then you have not diminished the light.
Is it open? You have not increased the light. By this illustration, brethren, understand that if the soul is pious, there is piety with God, of which the soul is partaker; if the soul is chaste, there is chastity with God, of which it partakes; if it is good, there is goodness with God, of which it partakes; if it is true, there is truth with God, of which the soul is partaker. Whereof if the soul is no partaker, every man is false; and if every man may be false, no man is true of himself. But the true Father is true of Himself, for He begot the Truth.
It is one thing to say, That man is true, for he has taken in the truth: it is another, God is true, for He begot the Truth. See then how God is true—not by participating in, but by generating the Truth. I see you have understood me, and am glad. Let this suffice you today. The rest, according as He gives it, we shall expound when the Lord pleases.
Source: Tractates on the Gospel of John (New Advent)