38 Such is the meaning of this passage, Have I been so long time with you, and you have not known Me, Philip? He that has seen Me has seen the Father also. How do you say, Show us the Father? Do you not believe Me, that I am in the Father, and the Father is in Me? It is only the Word of God, of Whom we men are enabled, in our discourse concerning Divine things, to reason. All else that belongs to the Godhead is dark and difficult, dangerous and obscure. If any man propose to express what is known in other words than those supplied by God, he must inevitably either display his own ignorance, or else leave his readers' minds in utter perplexity.
The Lord, when He was asked to show the Father, said, He that has seen Me has seen the Father also. He that would alter this is an antichrist, he that would deny it is a Jew, he that is ignorant a Pagan. If we find ourselves in difficulty, let us lay the fault to our own reason; if God's declaration seem involved in obscurity, let us assume that our want of faith is the cause. These words state with precision that God is not solitary, and yet that there are no differences within the Divine nature.
For the Father is seen in the Son, and this could be the case neither if He were a lonely Being, nor yet if He were unlike the Son. It is through the Son that the Father is seen: and this mystery which the Son reveals is that They are One God, but not one Person. What other meaning can you attach to this saying of the Lord's, He that has seen Me has seen the Father also? This is no case of identity; the use of the conjunction also shows that the Father is named in addition to the Son.
These words, The Father also, are incompatible with the notion of an isolated and single Person. No conclusion is possible but that the Father was made visible through the Son, because They are One and are alike in nature. And, lest our faith in this regard should be left in any doubt, the Lord proceeded, How do you say, Show us the Father? The Father had been seen in the Son; how then could men be ignorant of the Father? What need could there be for Him to be shown?
Source: On the Trinity (New Advent)