43 John speaks thus— For we know that the Son of God has come, and was incarnate for us, and suffered, and rose again from the dead and took us for Himself, and gave us a good understanding that we may know Him that is true, and may be in His true Son Jesus Christ. He is true and is life eternal and our resurrection. Wisdom doomed to an evil end, void of the Spirit of God, destined to possess the spirit and the name of Antichrist, blind to the truth that the Son of God came to fulfil the mystery of our salvation, and unworthy in that blindness to perceive the light of that sovereign knowledge!
For this wisdom asserts that Jesus Christ is no true Son of God, but a creature of His, Who bears the Divine name by adoption. In what dark oracle of hidden knowledge was the secret learned? To whose research do we owe this, the great discovery of the day? Were you he that lay upon the bosom of the Lord? You he to whom in the familiar intercourse of love He revealed the mystery? Was it you that alone followed Him to the foot of the Cross? And while He was charging you to receive Mary as your Mother, did He teach you this secret, as the token of His peculiar love for yourself?
Or did you run to the Sepulchre, and reach it sooner even than Peter, and so gain this knowledge there? Or was it amid the throngs of angels, and sealed books whose clasps none can open, and manifold influences of the signs of heaven, and unknown songs of the eternal choirs, that the Lamb, your Guide, revealed to you this godly doctrine, that the Father is no Father, the Son no Son, nor nature, nor truth? For you transform all these into lies. The Apostle, by that most excellent knowledge that was granted him, speaks of the Son of God as true.
You assert His creation, proclaim His adoption, deny His birth. While the true Son of God is eternal life and resurrection to us, for him, in whose eyes He is not true, there is neither eternal life nor resurrection. And this is the lesson taught by John, the disciple beloved of the Lord.
Source: On the Trinity (New Advent)