21 Thus the Father works hitherto and the Son works. In Father and Son you have the names which express Their nature in relation to Each other. Note also that it is the Divine nature, that through which God works, that is working here. And remember, lest you fall into the error of imagining that the operation of two unlike natures is here described, how it was said concerning the blind man, But that the works of God may be made manifest in him, I must work the works of Him that sent Me. You see that in his case the work wrought by the Son is the Father's work; and the Son's work is God's work.
The remainder of the discourse which we are considering also deals with works; but my defence is at present only concerned with assigning the whole work to Both, and pointing out that They are at one in Their method of working, since the Son is employed upon that work which the Father does hitherto. The sanction contained in this fact that, by virtue of His Divine birth, the Father is working with Him in all that He does, will save us from supposing that the Lord of the Sabbath was doing wrong in working on the Sabbath.
His Sonship is not affected, for there is no confusion of His Divinity with the Father's, and no negation of it; His Godhead is not affected, for His Divine nature is untouched. Their unity is not affected, for no difference is revealed to sever Them; and Their unity is not presented in such a light as to contradict Their distinct existence. First recognise the Sonship of the Son; The Son can do nothing of Himself, but what He sees the Father do. Here His birth is manifest; because of it He can do nothing of Himself till He sees it being done.
He cannot be unbegotten, because He can do nothing of Himself; He has no power of initiation, and therefore He must have been born. But the fact that He can see the Father's works proves that He has the comprehension which belongs to the conscious Possessor of Divinity. Next, mark that He does possess this true Divine nature;— For what things soever He does, these also does the Son likewise. And now that we have seen Him endowed with the powers of that nature, note how this results in unity, how one nature dwells in the Two— That all men may honour the Son, even as they honour the Father.
And then, lest reflection on this unity entangle you in the delusion of a solitary and self-contained God, take to heart the mystery of the faith manifested in these words, He that honours not the Son honours not the Father Which has sent Him. The rage and cunning of heresy may do their worst; our position is impregnable. He is the Son, because He can do nothing of Himself; He is God, because, whatever the Father does, He does the same; They Two are One, because He is equal in honour to the Father and does the very same works; He is not the Father, because He is sent.
So great is the wealth of mysterious truth contained in this one doctrine of the birth! It embraces His name, His nature, His power, His self-revelation; for everything conveyed to Him in His birth must be contained in that nature from which His birth is derived. Into His nature no element of any substance different in kind from that of His Author is introduced, for a nature which springs from one nature only must be entirely one with that nature which is its parent. An unity is that which, containing no discordant elements, is one in kind with itself; an unity constituted through birth cannot be solitary; for solitude can have but a single occupant, while an unity constituted through birth implies the conjunction of Two.
Source: On the Trinity (New Advent)