John Gill

Then said they unto him, who art thou?.... That talks at this rate, and threatens with death, in case of unbelief; this they said with an haughty air, and in a scornful manner: and Jesus saith unto them, even the same that I said unto you from the beginning; meaning, either of this discourse, as that he was the light of the world, and which he continued to assert; or of his being had before the sanhedrim, when he affirmed that God was his Father, and by many strong arguments proved his divine sonship; or of his ministry, when by miracles, as well as doctrines, he made it to appear that he was he that was to come, the true Messiah; or who spake from the beginning to Moses, saying, I am that I am, hath sent thee, and to the church, and Jewish fathers in the wilderness; and who is that word that was from the beginning with God; and who is called the beginning, the first cause of all things, and of the creation of God; and some think this is intended here.