6 “Whoso honors not the Son, honors not the Father that sent Him.” This is a truth, and is plain. Since, then, “all judgment has He given to the Son,” as He said above, “that all may honor the Son, even as they honor the Father,” what if there be those who honor the Father and honor not the Son? It cannot be, says He: “Whoso honors not the Son, honors not the Father that sent Him.” One cannot therefore say, I honored the Father, because I knew not the Son. If you did not yet honor the Son, neither did you honor the Father.
For what is honoring the Father, unless it be in that He has a Son? It is one thing when you are taught to honor God in that He is God; but another thing when you are taught to honor Him in that He is Father. When you are taught to honor Him in that He is God, it is as the Creator, as the Almighty, as the Spirit supreme, eternal, invisible, unchangeable, that you are led to think of Him; but when you are taught to honor Him in that He is Father, it is the same thing as to honor the Son; because Father cannot be said if there be not a Son, as neither can Son if there be not a Father.
But lest, it may be, you honor the Father indeed as greater, but the Son as less—as you may say to me, “I do honor the Father, for I know that He has a Son; nor do I err in the name Father, for I do not understand Father without Son, and yet the Son also I honor as the less,”— the Son Himself sets you right, and recalls you, saying, “that all may honor the Son,” not in a lower degree, but “as they honor the Father.” Therefore, “whoso honors not the Son, honors not the Father that sent Him.”
“I,” do you say, “wish to give greater honor to the Father, less to the Son.” Therein you take away honor from the Father, wherein you give less to the Son. For, being thus minded, it must really seem to you that the Father either would not or could not beget a Son equal to Himself: if He would not, He lacked the will; if He could not, He lacked the ability. Do you not therefore see that, being thus minded, wherein you would give greater honor to the Father, therein you are reproachful to the Father? Wherefore, so honor the Son as you honor the Father, if you would honor both the Father and the Son.
Source: Tractates on the Gospel of John (New Advent)