Augustine of Hippo — as excerpted in the Catena Aurea on John 12:44-50

“(Tr. liv. 2) He signifies to them that He is more than He appears to be, (for to men He appeared but a man; His Godhead was hid.) Such as the Father is, such am I in nature and in dignity; He that believeth on Me, believeth not on Me, i. e. on that which He sees, but on Him that sent Me, i. e. on the Father. [1He that believes in the Father must believe in Him as the Father, i. e. must believe that He has a Son; and reversely, he who believes in the Son thereby believes in the Father.] And again, if any one thinks that God has sons by grace, but not a Son equal and coeternal with Himself, neither does he believe 2on the Father, who sent the Son; because what he believes on is not the Father who sent Him. (c. 3.). And to shew that He is not the Son, in the sense of one out of many, a son by grace, but the Only Son equal to the Father, He adds, And He that seeth Me, seeth Him that sent Me; so little difference is there between Me and Him that sent Me, that He that seeth Me, seeth Him. Our Lord sent His Apostles, yet none of them dared to say, He that believeth on Me. We believe an Apostle, but we do not believe on an Apostle. Whereas the Only Begotten says, He that believeth on Me, doth not believe on Me, but on Him that sent Me. Wherein He does not withdraw the believer’s faith from Himself, but gives him a higher object than the form of a servant, for that faith.”