6 Yesterday we commended it to your consideration, beloved, and said that the sentences of the Evangelist John, in which he narrates to us what he learned from the Lord, had not required to be discussed, were that possible, except the inventions of heretics had compelled us. Yesterday, then, we briefly intimated to you, beloved, that there are heretics who are called Patripassians, or Sabellians after their founder: these say that the same is the Father who is the Son; the names different, but the person one.
When He wills, say they, He is Father; when He wills, He is Son: still He is one. There are likewise other heretics who are called Arians. They indeed confess that our Lord Jesus Christ is the only Son of the Father; the one, Father of the Son; the other, Son of the Father; that He who is Father is not Son, nor He who is Son is Father; they confess that the Son was begotten, but deny His equality. We, namely, the catholic faith, coming from the doctrine of the apostles planted in us, received by a line of succession, to be transmitted sound to posterity—the catholic faith, I say, has, between both those parties, that is, between both errors, held the truth.
In the error of the Sabellians, He is only one; the Father and Son is the same person: in the error of the Arians, the Father and the Son are indeed different persons; but the Son is not only a different person, but different in nature. Thou midway between these, what do you say? You have shut out the Sabellian, shut out the Arian also. The Father is Father, the Son is Son; another person, not another in nature; for, “I and the Father are one,” which, so far as I could, I pressed on your thoughts yesterday.
When he hears that word, we are, let the Sabellian go away confounded; when he hears the word one, let the Arian go away confounded. Let the catholic steer the bark of his faith between both, since in both he must be on his guard against shipwreck. Say thou, then, what the Gospel says, “I and the Father are one.” Not different in nature, because one; not one person, because are.
Source: Tractates on the Gospel of John (New Advent)