When our Lord said, He is true that sent Me, the Jews did not understand that He spake to them of the Father. But He saw some there, who, He knew, would believe on Him after His passion. Then said Jesus unto them, When ye have lifted up the Son of man, then ye shall know that I am. (Exod. 3:14) Recollect the words, I am that I am, and ye will know why I say, I am. I pass over your knowledge, in order that I may fulfil My passion. In your appointed time ye will know who I am; when ye have lifted up the Son of man. He means the lifting up of the cross; for He was lifted up on the cross, when He hung thereon. This was to be accomplished by the hands of those who should afterwards believe, whom He is now speaking to; with what intent, but that no one, however great his wickedness and consciousness of guilt might despair, seeing even the murderers of our Lord forgiven.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: Tract. xl. 2