8 The Apostle, who knew this mystery, and had received the knowledge of the faith through the Lord Himself, was not unmindful, that neither the world, nor mankind, nor philosophy could contain Him, for he writes, 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 Jesus Christ, for in Him dwells all the fullness of the Godhead bodily, and in Him you are made full, Who is the head of all principalities and powers. After the announcement that in Christ dwells all the fullness of the Godhead bodily, follows immediately the mystery of our assumption, in the words, in Him you are made full.
As the fullness of the Godhead is in Him, so we are made full in Him. The Apostle says not merely you are made full, but, in Him you are made full; for all who are, or shall be, regenerated through the hope of faith to life eternal, abide even now in the body of Christ; and afterwards they shall be made full no longer in Him, but in themselves, at the time of which the Apostle says, Who shall fashion anew the body of our humiliation, that it may be conformed to the body of His glory. Now, therefore, we are made full in Him, that is, by the assumption of His flesh, for in Him dwells the fullness of the Godhead bodily.
Nor has this our hope a light authority in Him. Our fullness in Him constitutes His headship and principality over all power, as it is written, That in His name every knee should bow, of things in heaven, and things on earth, and things below, and every tongue confess that Jesus is Lord in the glory of God the Father. Jesus shall be confessed in the glory of God the Father, born in man, yet now no longer abiding in the infirmity of our body, but in the glory of God. Every tongue shall confess this.
But though all things in heaven and earth shall bow the knee to Him, yet herein He is head of all principalities and powers, that to Him the whole universe shall bow the knee in submission, in Whom we are made full, Who through the fullness of the Godhead dwelling in Him bodily, shall be confessed in the glory of God the Father.
Source: On the Trinity (New Advent)