Theophylact of Ohrid

Do you see the equality of authority, and how He presents Himself as equally trustworthy as the Father? He would not have dared to say this if He were lower in dignity than the Father and were not equal to Him and Consubstantial. For if He had wanted to show that He is in any way inferior to the Father and less than Him, He would not have numbered Himself with the Father, nor would He have placed His own testimony alongside the testimony of the Father, but, being a slave, as the heretics blasphemously claim, He would have turned to …

Commentary on John ↗

Augustine of Hippo

"I am one that bear witness of myself, and the Father that sent me beareth witness of me." Therefore, because it is written in your law that the witness of two men is true, receive our witness, lest ye feel our judgment. "For I," saith He, "judge not any man; but I bear witness of myself:" I defer judgment, I defer not the witness. Let us, brethren, choose for ourselves God as our judge, God as our witness, against the tongues of men, against the weak suspicions of mankind. …

Tractates on John 36 ↗

Bede

In many places the Father bears witness of the Son; as, This day have I begotten Thee; (Ps. 2) also, This is My beloved Son. (Matt. 3:17)

Catena Aurea by Aquinas ↗

Thomas Aquinas

If, therefore, the testimony of two or three is true, my testimony is true, because It is I who bear witness to myself and the Father who sent me who bears witness concerning me: "I have testimony that is greater than that of John" (5:36). But this does not seem to be to the point. First, because the Father of the Son of God is not a man, while Christ says, the testimony of two men is true. …

Commentary on John ↗