5 Unlearned in the teaching of the Gospels and Apostles, they extol the glory of God the Father, not, however, with the sincerity of a devout believer, but with the cunning of impiety, to wrest from it an argument for their wicked heresy. Nothing, they say, can be compared with His nature: therefore the Only-begotten God is excluded from the comparison, because He possesses a lower and weaker nature. And this they say of God, the living image of the living God, the perfect form of His blessed nature, the only-begotten offspring of His unbegotten substance; Who is not truly the image of God unless He possesses the perfect glory of the Father's blessedness: and reproduces in its exactitude the likeness of His whole nature.
But if the Only-begotten God is the image of the Unbegotten God, the verity of that perfect and supreme nature resides in Him and makes Him the image of the very God. Is the Father omnipotent? The weak Son is not the image of omnipotence. Is He good? The Son, Whose divinity is of a lower stamp, does not reflect in His sinful nature the image of goodness. Is He incorporeal? The Son, Whose very spirit is confined to the limits of a body, is not in the form of the Incorporeal. Is He ineffable?
The Son, Whom language can define, Whose nature the tongue can describe, is not the image of the Ineffable. Is He the true God? The Son possesses only a fictitious divinity, and the false cannot be the image of the True. The Apostle, however, does not ascribe to Christ a portion of the image, or a part of the form, but pronounces Him unreservedly the image of the invisible God and the form of God. And how could He declare more expressly the divine nature of the Son of God, than by saying that Christ is the image of the invisible God even in respect of His invisibility: for if the substance of Christ were discernible how could He be the image of an invisible nature?
Source: On the Trinity (New Advent)