10 I shall speak, then, to our Lord Jesus Christ; I shall speak and may He be pleased to hear me. I believe He is present, I am fully assured of it; for He Himself has said, “Lo, I am with you even to the end of the world.” O Lord our God, what is that which Thou said, “If you believe not that I am”? For what is there that belongs not to the things You have made? Does not heaven so belong? Does not the earth? Does not everything in earth and heaven? Does not man himself to whom You speak?
Does not the angel whom You send? If all these are things made by You, what is that existence You have retained as something exclusively Your own, which You have given to none besides, that You might be such Yourself alone? For how do I hear “I am who am,” as if there were none besides? And how do I hear “If you believe not that I am”? For had they no existence who heard Him? Yea, though they were sinners, they were men. What then can I do? What that existence is, let Him tell my heart, let Him tell, let Him declare it within; let the inner man hear, the mind apprehend this true existence; for such existence is always unvarying in character. For a thing, anything whatever (I have begun as it were to dispute, and have left off inquiring.
Perhaps I wish to speak what I have heard. May He grant enlargement to my hearing, and to yours, while I speak)—for anything, whatever in short be its excellence, if it is changeable, does not truly exist; for there is no true existence wherever non-existence has also a place. For whatever can be changed, so far as changed, it is not that which was: if it is no longer what it was, a kind of death has therein taken place; something that was there has been eliminated, and exists no more.
Blackness has died out in the silvery locks of the patriarch, comeliness in the body of the careworn and crooked old man, strength in the body of the languishing, the [previous] standing posture in the body of one walking, walking in the body of one standing, walking and standing in the body of one reclining, speech in the tongue of the silent—whatever changes, and is what it was not, I see there a kind of life in that which is, and death in that which was. In fine, when we say of one deceased, Where is that person?
We are answered, He was. O Truth, it is thou [alone] that truly art! For in all actions and movements of ours, yea, in every activity of the creature, I find two times, the past and the future. I seek for the present, nothing stands still: what I have said is no longer present; what I am going to say is not yet come: what I have done is no longer present; what I am going to do is not yet come: the life I have lived is no longer present; the life I have still to live is not yet come.
Past and future I find in every creature-movement: in truth, which is abiding, past and future I find not, but the present alone, and that unchangeably, which has no place in the creature. Sift the mutations of things, you will find was and will be: think on God, you will find the is, where was and will be cannot exist. To be so then yourself, rise beyond the boundaries of time. But who can transcend the powers of his being? May He raise us there who said to the Father, “I will that they also be with me where I am.”
And so, in making this promise, that we should not die in our sins, the Lord Jesus Christ, I think, said nothing else by these words, “If you believe not that I am;” yea, by these words I think He meant nothing else than this, “If you believe not that I am” God, “you shall die in your sins.” Well, God be thanked that He said, “If you believe not,” and did not say, If you comprehend not. For who can comprehend this? Or is it so, since I have ventured to speak and you have seemed to understand, that you have indeed comprehended somewhat of a subject so unspeakable? If then you comprehend not, faith sets you free. Therefore also the Lord said not, If you comprehend not that I am; but said what they were capable of attaining, “If you believe not that I am, you shall die in your sins.”
Source: Tractates on the Gospel of John (New Advent)