30 They are blankly ignorant who separate the Divine name from the Divine nature; ignorant, and content to be ignorant. But let them listen to the reproof which the Son inflicts upon unbelievers for their want of this knowledge, when the Jews said that God was their Father:— If God were your Father, you would surely love Me; for I went forth from God, and have come; neither am I come of Myself, but He sent Me. The Son of God has here no word of blame for the devout confidence of those who combine the confession that He is true God, the Son of God, with their own claim to be God's sons.
What He is blaming is the insolence of the Jews in daring to claim God as their Father, when meanwhile they did not love Him, the Son:— If God were your Father, you would surely love Me; for I went forth from God. All, who have God for their Father through faith, have Him for Father through that same faith whereby we confess that Jesus Christ is the Son of God. But to confess that He is the Son in a sense which covers the whole company of saints; to say, in effect, that He is one of the sons of God—what faith is there in that?
Are not all the rest, feeble created beings though they be, in that sense sons? In what does the eminence of a faith, which has confessed that Jesus Christ is the Son of God, consist, if He, as one of a multitude of sons, have the name only, and not the nature, of the Son? This unbelief has no love for Christ; it is a mockery of the faith for these perverters of the truth to claim God as their Father. If He were their Father, they would love Christ because He had gone forth from God.
And now I must enquire the meaning of this going forth from God. His going forth is obviously different from His coming, for the two are mentioned side by side in this passage, I went forth from God and have come. In order to elucidate the separate meanings of I went forth from God and I have come, He immediately subjoins, Neither am I come of Myself, but He sent Me. He tells us that He is not the source of His own existence in the words, Neither am I come of Myself.
In them He tells us that He has proceeded forth a second time from God, and has been sent by Him. But when He tells us that they who call God their Father must love Himself because He has gone forth from God, He makes His birth the reason for their love. Went forth carries back our thoughts to the incorporeal birth, for it is by love of Christ, Who was born from Him, that we must gain the right of devoutly claiming God for our Father. For when the Son says, He that hates Me hates My Father also, this My is the assertion of a relation to the Father which is shared by none.
On the other hand, He condemns the man who claims God as his Father, and loves not the Son, as using a wrongful liberty with the Father's name; since he who hates Him, the Son, must hate the Father also, and none can be devoted to the Father save those who love the Son. For the one and only reason which He gives for loving the Son is His origin from the Father. The Son, therefore, is from the Father, not by His Advent, but by His birth; and love for the Father is only possible to those who believe that the Son is from Him.
Source: On the Trinity (New Advent)