6 But see also what He says of that future judgment in the end: “He that despises me, and receives not my words, has one that judges him: the word that I have spoken, the same shall judge him in the last day.” He says not, He that despises me, and receives not my words, I judge him not at the last day; for had He said so, I do not see how it could have been else than contradictory of that other statement, when He says, “The Father judges no man, but has committed all judgment unto the Son.”
But when He said, “He that despises me, and receives not my words, has one to judge him,” and, for the information of those who were waiting to hear who that one was, went on to add, “The word that I have spoken, the same shall judge him in the last day,” He made it sufficiently manifest that He Himself would then be the judge. For it was of Himself He spoke, Himself He announced, and Himself He set forth as the gate whereby He entered as the Shepherd to His sheep. In one way, therefore, will those be judged who have never heard that word, in another way those who have heard and despised. “For as many as have sinned without law,” says the apostle, “shall also perish without law; and as many as have sinned in the law, shall be judged by the law.”
Source: Tractates on the Gospel of John (New Advent)