5 Sabellius sweeps away the birth of the Son, and then preaches the unity of God; but he does not doubt that the mighty Nature, which acted in the human Christ, was God. He shuts his eyes to the revealed mystery of the Sonship; the works done seem to him so marvellous that he cannot believe that He who performed them could undergo a true generation. When he hears the words, He that has seen Me has seen the Father also, he jumps to the blasphemous conclusion of an inseparable and indistinguishable identity of nature in Father and Son, because he fails to see that the revelation of the birth is the mode in which Their unity of nature is manifested to us.
For the fact that the Father is seen in the Son is a proof of the Son's Divinity, not a disproof of His birth. Thus our knowledge of Each of Them is conditioned by our knowledge of the Other, for there is no difference of nature between them and, since in this respect they are One, a reverent study of the character of Either will give us a true insight into the nature of Both. For, indeed, it is certain that He, Who was in the form of God, must in His self-revelation present Himself to us in the exact aspect of the form of God. Again, this perverse and insane delusion derives a further encouragement from the words, I and the Father are One. From the fact of unity in the same nature they have impiously deduced a confusion of Persons; their interpretation, that the words signify a single Power, contradicts the tenour of the passage.
For I and the Father are One does not indicate a solitary God. The use of the conjunction and shows clearly that more than one Person is signified; and are requires a plurality of subject. Moreover, the One is not incompatible with a birth. Its sense is, that the Two Persons have the one nature in common. The One is inconsistent with difference; the are with identity.
Source: On the Trinity (New Advent)