When ye have lifted up - When you have crucified. See John 3:14 note; also John 12:32 note.
The Son of man - See the notes at Matthew 8:19-20.
Then shall ye know - Then shall you 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.