I recall hearing an explanation once about the role of an ambassador. A head of state sends an ambassador to a foreign country to deliver a message. The ambassador speaks with the voice of his government. He does not speak his own thoughts or opinions, unless he has been given permission to do so for the sake of negotiations. The foreign head of state can rely on the ambassador to deliver an accurate position of where the ambassador’s government stands on the issues. The ambassador has an obligation to deliver that message accurately and faithfully, even when he might disagree with certain details. For the ambassador this is his job.
Our Lord is not an ambassador. The message is not apart from Him. Rather He speaks a message directly from the Father. He is the message. As He said earlier in the Gospel of St. John, “The Father and I are one” (John 10:30). Our Lord frames this reference to His identity in the form of a warning to those who would reject Him. “Whoever rejects me and does not accept my words has something to judge him: the word that I spoke, it will condemn him on the last day, because I did not speak on my own, but the Father who sent me commanded me what to say and speak” (John 12:48-49).
The identification between the Son and the Father is a profound one. St. Augustine offers a beautiful observation about this passage from the Gospel of St. John: “For the word that He speaks, is Himself. He speaks Himself, announces Himself” (Tr. Liv. 6). He is the Word of God made flesh.