3 The Lord then, recognizing such in that crowd, said, “When you have lifted up the Son of man, then shall you know that I am [He].” You know already what “I am” signifies; and we must not be continually repeating, lest so great a subject beget distaste. Recall that, “I am who am,” and “He who is has sent me,” and you will recognize the meaning of the words, “Then shall you know that I am.” But both the Father is, and the Holy Spirit is. To the same is belongs the whole Trinity.
But because the Lord spoke as the Son, in order that, when He says, “Then shall you know that I am,” there might be no chance of entrance for the error of the Sabellians, that is, of the Patripassians,— an error which I have charged you not to hold, but to beware of—the error, I mean, of those who have said, The Father and Son are one and the same; two names, but one reality—to guard them against that error, when the Lord said, “Then shall you know that I am,” that He might not be understood as Himself the Father, He immediately added, “And I do nothing of myself; but as my Father taught me, I speak these things.”
Already was the Sabellian beginning to rejoice over the discovery of a ground for his error; but immediately on showing himself as it were in the shade, he was confounded by the light of the following sentence. You thought that He was the Father, because He said, “I am.” Hear now that He is the Son: “And I do nothing of myself.” What means this, “I do nothing of myself”? Of myself I am not. For the Son is God, of the Father; but the Father is God, yet not of the Son. The Son is God of God, and the Father is God, but not of God. The Son is light of light; and the Father is light, but not of light. The Son is, but there is [One] of whom He is; and the Father is, but there is none of whom He is.
Source: Tractates on the Gospel of John (New Advent)