5 He, then, by whom all things were made knows all things, and yet He rebukes by doubting: “If you knew me ye would perhaps know my Father also.” He rebukes unbelievers. He spoke a like sentence to the disciples, but there is not a word of doubting in it, because there was no occasion to rebuke unbelief. For this, “If you knew me, you would perhaps know my Father also,” which He said to the Jews, He said also to the disciples, when Philip asked, or rather, demanded of Him, saying, “Lord, show us the Father, and it suffices us:” just as if he said, We already know You even ourselves; You have been apparent to us; we have seen You; You have deigned to choose us; we have followed You, have seen Your marvels, heard Your words of Salvation, have taken Your precepts upon us, we hope in Your promises: You have deigned to confer much upon us by Your very presence: but still, while we know You, and we do not yet know the Father, we are inflamed with desire to see Him whom we do not yet know; and thus, be cause we know You, but it is not enough until we know the Father, show us the Father and it suffices us. And the Lord, that they might understand that they knew not what they thought they did already know, said, “Am I so long time with you, and you know me not, Philip? He who has seen me has seen the Father.” Has this sentence a word of doubting in it? Did He say, He that has seen me has perhaps seen the Father? Why not? Because it was a believer that listened to Him, not a persecutor of the faith: hence did the Lord not rebuke, but teach. “Whoso has seen me has seen the Father also:” and here, “If you knew me, you would know my Father also,” let us remove the word which indicates the unbelief of the hearers, and it is the same sentence.
Source: Tractates on the Gospel of John (New Advent)