I and My Father are one, not only by agreement and consent of will, as the Arians hold, but also one in Essence and Godhead, the same in number,* not in species, for otherwise there would be more Gods than one. Christ speaks here as God and the Word of the Father. And from this the fathers prove His Godhead against the Arians. And the Jews understood the words in the same sense, and consequently sought to stone Him as a blasphemer. And Christ Himself explained them in the same sense, for He said, I am the Son of God. It is clear also from His line of argument, "being one with the Father I have the same Almighty power." For where the essence is the same, the power is also the same. So says S. Hilary (de Trinit. lib. viii.), "The Father and the Son are One, not as He speaks of the faithful (in chap. xvii.), "That they may be one," but one in nature, honour, and power." "He steers between Scylla and Charybdis," says S. Augustine (in loc.) "between Arius and Sabellius; for by speaking of "One" He signifies Oneness of nature. But by saying "we are" He indicates a plurality of persons, which Sabellius denied, affirming that God was One in Person, as well as in Essence." S. Augustine says the same (de Trinit. vi2). See Bellarmine (de Christo, i6).
