43 But what absurd folly is it of the heretics to regard as unattainable for God that goal to which man hopes to attain, to imply that He is powerless to effect in Himself that which He is mighty to effect in us. It is not the language of reason or common sense to say that God is bound by some necessity of His nature to consult our happiness, but cannot bestow the like blessings upon Himself. God does not, indeed, need any further blessedness, for His nature and power stand fast in their eternal perfection.
But although in the Dispensation, that mystery of great godliness, He Who is God became man, He is not powerless to make Himself again entirely God, for without doubt He will transform us also into that which as yet we are not. The final sequel of man's life and death is the resurrection: the assured reward of our warfare is immortality and incorruption, not the ceaseless persistence of everlasting punishment, but the unbroken enjoyment and happiness of eternal glory. These bodies of earthly origin shall be exalted to the fashion of a higher nature, and conformed to the glory of the Lord's body.
But what then of God found in the form of a servant? Though already, while still in the form of a servant, glorified in the body, shall He not be also conformed to God? Shall He bestow upon us the form of His glorified body, and yet be able to do for His own body nothing more than He does for Himself in common with us? For the most part the heretics interpret the words, Then shall He be subjected to Him that did subject all things to Himself, that God may be all in all, as if they meant that the Son is to be subjected to God the Father, in order that by the subjection of the Son, God the Father may be all in all.
But is there still lacking in God some perfection which He is to obtain by the subjection of the Son? Can they believe that God does not already possess that final accession of blessed divinity, because it is said that by the coming of the fullness of time He shall be made all in all?
Source: On the Trinity (New Advent)