50 He is accordingly the first-born of every creature because in Him all things were created. And lest any one should dare to refer to any other than Him the creation of all things in Himself, he says, All things have been created through Him and in Him, and He is before all, and for Him all things consist. All things then consist for Him Who is before all things, and in Whom are all things. Now this indeed describes the origin of created things. But concerning the dispensation by which He assumed our body, he adds, And He is the head of the body, the Church: Who is the beginning, the first-born from the dead: that in all things He might have the pre-eminence.
For it was the good pleasure of the Father that in Him should all the fullness dwell, and that through Him all things should be reconciled to Him</em>. The Apostle has assigned to the spiritual mysteries their material effects. For He Who is the image of the invisible God is Himself the head of His body, the Church, and He Who is the first-born of every creature is at the same time the beginning, the first born from the dead: that in all things He might have the pre-eminence, being for us the Body, while He is also the image of God, since He, Who is the first-born of created things, is at the same time the first-born for eternity; so that as to Him things spiritual, being created in the First-born, owe it that they abide, even so all things human also owe it to Him that in the First-born from the dead they are born again into eternity.
For He is Himself the beginning, Who as Son is therefore the image, and because the image, is of God. Further He is the first-born of every created thing, possessing in Himself the origin of the universe: and again He is the head of His body, the Church, and the first-born from the dead, so that in all things He has the pre-eminence. And because all things consist for Him, in Him the fullness of the Godhead is pleased to dwell, for in Him all things are reconciled through Him to Him, through Whom all things were created in Himself.
Source: On the Trinity (New Advent)