7 If, again, he allege His own word when He said, “I and the Father are one,” let him attend to the fact, and understand that He did not say, “I and the Father am one, but are one.” For the word are is not said of one person, but it refers to two persons, and one power. He has Himself made this clear, when He spoke to His Father concerning the disciples, “The glory which You gave me I have given them; that they may be one, even as we are one: I in them, and You in me, that they may be made perfect in one; that the world may know that You have sent me.” What have the Noetians to say to these things?
Are all one body in respect of substance, or is it that we become one in the power and disposition of unity of mind? In the same manner the Son, who was sent and was not known of those who are in the world, confessed that He was in the Father in power and disposition. For the Son is the one mind of the Father. We who have the Father's mind believe so (in Him); but they who have it not have denied the Son. And if, again, they choose to allege the fact that Philip inquired about the Father, saying, “Show us the Father, and it suffices us,” to whom the Lord made answer in these terms: “Have I been so long time with you, and yet have you not known me, Philip?
He that has seen me has seen the Father. Do you not believe that I am in the Father, and the Father in me?” and if they choose to maintain that their dogma is ratified by this passage, as if He owned Himself to be the Father, let them know that it is decidedly against them, and that they are confuted by this very word. For though Christ had spoken of Himself, and showed Himself among all as the Son, they had not yet recognised Him to be such, neither had they been able to apprehend or contemplate His real power.
And Philip, not having been able to receive this, as far as it was possible to see it, requested to behold the Father. To whom then the Lord said, “Philip, have I been so long time with you, and yet have you not known me? He that has seen me has seen the Father.” By which He means, If you have seen me, you may know the Father through me. For through the image, which is like (the original), the Father is made readily known. But if you have not known the image, which is the Son, how do you seek to see the Father? And that this is the case is made clear by the rest of the chapter, which signifies that the Son who “has been set forth was sent from the Father, and goes to the Father.”
Source: Against the Heresy of Noetus (New Advent)