Albert Barnes Commentary John 8:28

"Jesus therefore said, When ye have lifted up the Son of man, then shall ye know that I am [he], and [that] I do nothing of myself, but as the Father taught me, I speak these things." — John 8:28 (ASV)

When you have lifted up. When you have crucified.

The Son of man.

Then you shall know. Then you shall have evidence or proof.

That I am he. Am the Messiah, which I have professed to be.

And that I do nothing of myself. That is, you shall have proof that God has sent me, that I am the Messiah, and that God concurs with me and approves my doctrine. This proof was furnished by the miracles that attended the death of Jesus—the earthquake and darkness; but chiefly by his resurrection from the dead, which proved, beyond a doubt, that he was what he affirmed he was—the Messiah.