Theophylact of Ohrid

Does he who sees the physical features? No. For the Father is not a body, so that one could say that he who sees Christ bodily also sees the Father, but by seeing, I ask you, understand mental contemplation. The Lord speaks as if to say: "He who by contemplation of the mind has embraced My essence, insofar as is possible for man, has also embraced the essence of the Father. He who has recognized Me as God has, without doubt, also recognized the Father. …

Commentary on John ↗

Thomas Aquinas

Then when he says, and he who sees me sees him who sent me, he treats of vision. In regard to this we should note that just as the Father sent the Son to convert the Jews, so Christ also sent his disciples: "As the Father has sent me, even so I send you" (20:21). But no one of the disciples dared to say, nor should he, that one should believe in him, although he could say that one should believe him. …

Commentary on John ↗

Bonaventure

And he who sees me sees him who sent me: therefore the one sending and the one sent are one in essence, because he who sees the one sees also the other. Hence below in chapter fourteen: "Philip, he who sees me sees also my Father." And the reason is the unity of essence, "because I am in the Father, and the Father is in me"; and the Son himself is the means of seeing the Father, as the word is of seeing the speaker, as splendor is of seeing light.

Commentary on John, Chapter 12 ↗