1 In the last book we treated of the indistinguishable nature of God the Father and God the Son, and demonstrated that the words, I and the Father are One, go to prove not a solitary God, but a unity of the Godhead unbroken by the birth of the Son: for God can be born only of God, and He that is born God of God must be all that God is. We reviewed, although not exhaustively, yet enough to make our meaning clear, the sayings of our Lord and the Apostles, which teach the inseparable nature and power of the Father and the Son; and we came to the passage in the teaching of the Apostle, where he says, Take heed lest there shall be any one that leads you astray through philosophy and vain deceit, after the tradition of men, after the rudiments of the world, and not after Christ; for in Him dwells all the fullness of the Godhead bodily. We pointed out that here the words, in Him dwells all the fullness of the Godhead bodily, prove Him true and perfect God of His Father's nature, neither severing Him from, nor identifying Him with, the Father.
On the one hand we are taught that, since the incorporeal God dwelt in Him bodily, the Son as God begotten of God is in natural unity with the Father: and on the other hand, if God dwelt in Christ, this proves the birth of the personal Christ in Whom He dwelt. We have thus, it seems to me, more than answered the irreverence of those who refer to a unity or agreement of will such words of the Lord as, He that has seen Me has seen the Father, or, The Father is in Me and I in the Father, or, I and the Father are One, or, All things whatsoever the Father has are Mine. Not daring to deny the words themselves, these false teachers, in the mask of religion, corrupt the sense of the words.
For instance, it is true that where the unity of nature is proclaimed the agreement of will cannot be denied; but in order to set aside that unity which follows from the birth, they profess merely a relationship of mutual harmony. But the blessed Apostle, after many indubitable statements of the real truth, cuts short their rash and profane assertions, by saying, in Christ dwells all the fullness of the Godhead bodily, for by the bodily indwelling of the incorporeal God in Christ is taught the strict unity of Their nature.
It is, therefore, not a matter of words, but a real truth that the Son was not alone, but the Father abode in Him: and not only abode, but also worked and spoke: not only worked and spoke, but also manifested Himself in Him. Through the Mystery of the birth the Son's power is the power of the Father, His authority the Father's authority, His nature the Father's nature. By His birth the Son possesses the nature of the Father: as the Father's image, He reproduces from the Father all that is in the Father, because He is the reality as well as the image of the Father, for a perfect birth produces a perfect image, and the fullness of the Godhead dwelling bodily in Him indicates the truth of His nature.
Source: On the Trinity (New Advent)