"Abraham pater vester exsultavit ut videret diem meum: vidit, et gavisus est." Dixerunt ergo Iudæi ad eum: "Quinquaginta annos nondum habes, et Abraham vidisti?" Dixit eis Iesus: "Amen, amen dico vobis, antequam Abraham fieret, ego sum." (From the Gospel for Passion Sunday, John viii, 56-58: "Abraham your father rejoiced that he might see my day: he saw it, and was glad." The Jews therefore said to him: "Thou art not yet fifty years old, and hast thou seen Abraham?" Jesus said to them: "Amen, amen I say to you, before Abraham was made, I am.")
...Abraham's Creator bears a great testimony to Abraham. "Abraham rejoiced," He says, "to see my day." He did not fear, but "rejoiced to see it." For in him there was the love that casts out fear. He says not, rejoiced because he saw; but "rejoiced that he might see." [Non ait: Exsultavit, quia vidit; sed, exsultavit ut videret.] Believing, at all events, he rejoiced in hope to see with the understanding. "And he saw." And what more could the Lord Jesus Christ say, or what more ought He to have said? "And he saw," He says, "and was glad." Who can unfold this joy...? If those rejoiced whose bodily eyes were opened by the Lord, what joy was his who saw with the eyes of his soul the light ineffable, the abiding Word, the brilliance that dazzles the minds of the pious, the unfailing Wisdom, God abiding with the Father, and at some time come in the flesh and yet not to withdraw from the bosom of the Father?
All this did Abraham see. For in saying "my day," it may be uncertain of what He spoke; whether the day of the Lord in time, when He should come the flesh, or that day of the Lord which knows not a dawn, and knows no decline. But for my part I doubt not that father Abraham knew it all. And where shall I find it out? Ought the testimony of our Lord Jesus Christ to satisfy us? Let us suppose that we cannot find it out, for perhaps it is difficult to say in what sense it is clear that Abraham "rejoiced to see the day" of Christ, "and saw it, and was glad."
"Before Abraham was made, I am." Weigh the words, and get a knowledge of the mystery. "Before Abraham was made." Understand, that "was made" refers to human formation; but "am" to the Divine essence. "He was made," because Abraham was a creature. He did not say, Before Abraham was, I was; but, "Before Abraham was made," who was not made save by me, "I am." Nor did He say this, Before Abraham was made I was made; for "In the beginning God created the heaven and the earth;" Genesis 1:1 and "in the beginning was the Word." "Before Abraham was made, I am."
Recognize the Creator—distinguish the creature. [Agnoscite Creatorem, discernite creaturam.] He who spoke was made the seed of Abraham; and that Abraham might be made, He Himself was before Abraham.
In Evangelium Ioannis - Tractatus XLIII