3 And, accordingly, after saying, “He that believes in me, believes not on me, but on Him that sent me,” that it might not be thought that He would have the Father so understood, as if He were the Father only of many sons regenerated by grace, and not of the only-begotten Word, His own co-equal, He immediately added, “And he that sees me, sees Him that sent me.” Does He say here, He that sees me, sees not me, but Him that sent me, as He had said, “He that believes me, believes not on me, but on Him that sent me”?
For He uttered the former of these words, that He might not be believed on merely as He then appeared, that is, as the Son of man; and the latter, that He might be believed on as the equal of the Father. He that believes in me, believes not merely on what He sees of me, but believes in Him that sent me. Or, when he believes in the Father, who begot me, His own co-equal, let him believe in me, not as he sees me, but as [he believes] on Him that sent me; for so far does the truth, that there is no distance between Him and me, reach, that He who sees me, sees Him that sent me.
Certainly, Christ the Lord Himself sent His apostles, as their name implies: for as those who in Greek are called angeli are in Latin called nuntii [messengers], so the Greek apostoli [apostles] becomes the Latin missi [persons sent]. But never would any of the apostles have dared to say, “He that believes in me, believes not on me, but on Him that sent me;” for in no sense whatever would he say, “He that believes in me.” We believe an apostle, but we do not believe in him; for it is not an apostle that justifies the ungodly.
But to him that believes in Him that justifies the ungodly, his faith is counted for righteousness. An apostle might say, He that receives me, receives Him that sent me; or, He that hears me, hears Him that sent me; for the Lord tells them so Himself: “He that receives you, receives me; and he that receives me, receives Him that sent me.” For the master is honored in the servant, and the father in the son: but then the father is as it were in the son, and the master as it were in the servant.
But the only-begotten Son could rightly say, “Believe in God, and believe in me;” as also what He says here, “He that believes in me, believes not on me, but on Him that sent me.” He did not turn away the faith of the believer from Himself, but only would not have the believer continue in the form of a servant: because every one who believes in the Father that sent Him, straightway believes in the Son, without whom he knows that the Father has no existence as such, and thus reaches in his faith to the belief of His equality with the Father, in conformity with the words that follow, “And he that sees me, sees Him that sent me.”
Source: Tractates on the Gospel of John (New Advent)