37 And therefore the Lord reproached them that they had not known Him, though He had so long been doing these works, and answered their prayer that He would show them the Father by saying, He that has seen Me has seen the Father also. He was not speaking of a bodily manifestation, of perception by the eye of flesh, but by that eye of which He had once spoken— Say not ye, There are yet four months, and then comes harvest? Behold, I say unto you, Lift up your eyes and look on the fields; for they are white to harvest. The season of the year, the fields white to harvest are allusions equally incompatible with an earthly and visible prospect.
He was bidding them lift the eyes of their understanding to contemplate the bliss of the final harvest. And so it is with His present words, He that has seen Me has seen the Father also. It was not the carnal body, which He had received by birth from the Virgin, that could manifest to them the image and likeness of God. The human aspect which He wore could be no aid towards the mental vision of the incorporeal God. But God was recognised in Christ, by such as recognised Christ as the Son on the evidence of the powers of His Divine nature; and a recognition of God the Son produces a recognition of God the Father.
For the Son is in such a sense the Image, as to be One in kind with the Father, and yet to indicate that the Father is His Origin. Other images, made of metals or colours or other materials by various arts, reproduce the appearance of the objects which they represent. Yet can lifeless copies be put on a level with their living originals? Painted or carved or molten effigies with the nature which they imitate? The Son is not the Image of the Father after such a fashion as this; He is the living Image of the Living.
The Son that is born of the Father has a nature in no wise different from His; and, because His nature is not different, He possesses the power of that nature which is the same as His own. The fact that He is the Image proves that God the Father is the Author of the birth of the Only-begotten, Who is Himself revealed as the Likeness and Image of the invisible God. And hence the likeness, which is joined in union with the Divine nature, is indelibly His, because the powers of that nature are inalienably His own.
Source: On the Trinity (New Advent)