What does John 8:25-28 mean?

25 Then said they unto him, Who art thou? And Jesus saith unto them, Even the same that I said unto you from the beginning. 26 I have many things to say and to judge of you: but he that sent me is true; and I speak to the world those things which I have heard of him. 27 They understood not that he spake to them of the Father. 28 Then said Jesus unto them, 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 my Father hath taught me, I speak these things. (John 8:25-28 KJV)

William Burkitt’s Commentary

The Jews hearing our Saviour denouncing such a terrible threatening against them, because they believed not on him in the foregoing verses; here they perversely ask him, Who he was? Our Saviour replies, That he was the same that he was from the beginning, even the very Christ; and that they were the very same, they were also the mortal enemies and opposers of the truth. But that the time was hastening, when they should be fully convinced who he was; namely, when they had lifted him up upon his cross, when he was risen again, and ascended into heaven, and brought that destruction upon them, which he had so often threatened.

Learn hence, That the sufferings of Christ were clear and convincing demonstrations, both who he was, and what he was. The darkening of the sun, the quaking of the earth, the rending of the rocks, the opening of the graves, were such convincing proofs of his deity, that they could not but say, Verily, this was the Son of God.