26 Once more, God the Only-begotten has summed up for us, in words of His own, the whole revealed mystery of the faith. When He had given His answer to the charge that He, being a man, made Himself God, He determined to show that His words, I and the Father are One, are a clear and necessary conclusion; and therefore He thus pursued His argument— You say that I have blasphemed, because I said, I am the Son of God. If I do not the works of the Father, believe Me not; but if I do, and you will not believe Me, believe the works, that you may know and be sure that the Father is in Me, and I in the Father.
After this, heresy that still persists in its course perpetrates a wilful outrage in conscious despair; the assertion of unbelief is deliberate shamelessness. They who make it take pride in folly and are dead to the faith, for it is not ignorance, but madness, to contradict this saying. The Lord had said, I and the Father are One; and the mystery of His birth, which He revealed, was the unity in nature of Father and Son. Again, when He was accused for claiming the Divine nature, He justified His claim by advancing a reason;— If I do not the works of the Father, believe Me not.
We are not to believe His assertion that He is the Son of God, unless He does His Father's works. Hence we see that His birth has given Him no new or alien nature, for His doing of the Father's works is to be the reason why we must believe that He is the Son. What room is there here for adoption, or for leave to use the name, or for denial that He was born from the nature of God, when the proof that He is God's Son is that He does the works which belong to the Father's nature?
No creature is equal or like to God, no nature external to His is comparable in might to Him; it is only the Son, born from Himself, Whom we can without blasphemy liken and equal to Him. Nothing outside Himself can be compared to God without insult to His august majesty. If any being, not born from God's self, can be discovered that is like Him and equal to Him in power, then God, in admitting a partner to share His throne, forfeits His pre-eminence. No longer is God One, for a second, indistinguishable from Himself, has arisen.
On the other hand, there is no insult in making His own true Son His equal. For then that which is like Him is His own; that which is compared with Him is born from Himself; the Power that can do His own works is not external to Him. Nay more, it is an actual heightening of His glory, that He has begotten Omnipotence, and yet not severed that Omnipotent nature from Himself. The Son performs the Father's works, and on that ground demands that we should believe that He is God's Son.
This is no claim of mere arrogance; for He bases it upon His works, and bids us examine them. And He bears witness that these works are not His own, but His Father's. He would not have our thoughts distracted by the splendour of the deeds from the evidence for His birth. And because the Jews could not penetrate the mystery of the Body which He had taken, the Humanity born of Mary, and recognise the Son of God, He appeals to His deeds for confirmation of His right to the name— But if I do them, and you will not believe Me, believe the works.
First, He would not have them believe that He is the Son of God, except on the evidence of God's works which He does. Next, if He does the works, yet seems unworthy, in His bodily humility, to bear the Divine name, He demands that they shall believe the works. Why should the mystery of His human birth hinder our recognition of His birth as God, when He that is Divinely born fulfils every Divine task by the agency of that Manhood which He has assumed? If we believe not the Man, for the works' sake, when He tells us that He is the Son of God, let us believe the works when they, which are beyond a doubt the works of God, are manifestly wrought by the Son of God.
For the Son of God possesses, in virtue of His birth, everything that is God's; and therefore the Son's work is the Father's work because His birth has not excluded Him from that nature which is His source and wherein He abides, and because He has in Himself that nature to which He owes it that He exists eternally.
Source: On the Trinity (New Advent)