Church Fathers on John 12:49

Because the love of human praise prevented the chief rulers from believing, Jesus cried and said, He that believes in Me, believes not on Me, but on Him that sent Me: as if to say, Why are you afraid to believe in Me? Your faith through Me passes to God.

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 believes in Me, believes not in Me, i.e. on that which He sees, but in Him that sent Me, i.e. on the Father. [He that believes in the Father must believe in Him as the Father, i.e. must believe that He has a Son; and reversibly, he who believes in the Son thereby believes in the Father.] And…

Since the Son is the Word of the Father, and reveals completely what is in the mind of the Father, He says He receives a commandment what He should say, and what He should speak: just as our word, if we say what we think, brings out what is in our minds.