1 The words of our Lord Jesus Christ, which He had addressed to the Jews, so regulating His discourse that the blind saw not, and believers' eyes were opened, are these, which have been read today from the holy Gospel: “Then said the Jews, Who are you?” Because the Lord had said before, “If you believe not that I am, you shall die in your sins.” To this accordingly they rejoined, “Who are you?” as if seeking to know on whom they ought to believe, so as not to die in their sin.
He replied to those who asked Him: “Who are you?” by saying, “The beginning, for [so] also I speak to you.” If the Lord has called Himself the beginning, it may be inquired whether the Father also is the beginning. For if the Son who has a Father is the beginning, how much more easily must God the Father be understood as the beginning, who has indeed the Son whose Father He is, but has no one from whom He Himself proceeds? For the Son is the Son of the Father, and the Father certainly is the Father of the Son; but the Son is called God of God—the Son is called Light of Light; the Father is called Light, but not, of Light—the Father is called God, but not, of God.
If, then, God of God, Light of Light, is the beginning, how much more easily may we understand as such that Light, from whom the Light [comes], and God, of whom is God? It seems, therefore, absurd, dearly beloved, to call the Son the beginning, and not to call the Father the beginning also.
Source: Tractates on the Gospel of John (New Advent)