2 We have spoken to you on the preceding passage, suggesting how the Father may be understood as True, and the Son as the Truth. But when the Lord Jesus said, “He that sent me is true,” the Jews understood not that He spoke to them of the Father. And He said to them, as you have just heard in the reading, “When you have lifted up the Son of man, then shall you know that I am, and [that] I do nothing of myself; but as the Father has taught me, I speak these things.” What means this?
For it looks as if all He said was, that they would know who He was after His passion. Without doubt, therefore, He saw that some there, whom He Himself knew, whom with the rest of His saints He Himself in His foreknowledge had chosen before the foundation of the world, would believe after His passion. These are the very persons whom we are constantly commending, and with much entreaty setting forth for your imitation. For on the sending down of the Holy Spirit after the Lord's passion, and resurrection, and ascension, when miracles were being done in the name of Him whom, as if dead, the persecuting Jews had despised, they were pricked in their hearts; and they who in their rage slew Him were changed and believed; and they who in their rage shed His blood, now in the spirit of faith drank it; to wit, those three thousand, and those five thousand Jews whom now He saw there, when He said, “When you have lifted up the Son of man, then shall you know that I am [He].”
It was as if He had said, I let your recognition lie over till I have completed my passion: in your own order you shall know who I am. Not that all who heard Him were only then to believe, that is, after the Lord's passion; for a little after it is said, “As He spoke these words, many believed on Him;” and the Son of man was not yet lifted up. But the lifting up He is speaking of is that of His passion, not of His glorification; of the cross, not of heaven; for He was exalted there also when He hung on the tree.
But that exaltation was His humiliation; for then He became obedient even to the death of the cross. This required to be accomplished by the hands of those who should afterwards believe, and to whom He says, “When you have lifted up the Son of man, then shall you know that I am [He].” And why so, but that no one might despair, however guilty his conscience, when he saw those forgiven their homicide who had slain the Christ?
Source: Tractates on the Gospel of John (New Advent)