49 But, as it is, neither did the Lord leave us in doubt: He who has seen Me, has seen the Father also; nor was the Apostle silent as to His nature, Who is the image of the invisible God. For the Lord had said, If I do not the works of My Father, believe Me not, teaching them to see the Father in Himself in that He did the works of the Father; that through perceiving the power of His nature they might understand the nature of that power which they perceived.
Wherefore the Apostle proclaiming that this is the image of God, says, Who is the image of the invisible God, the first-born of all creation; for in Him were all things made in the heavens and upon the earth, things visible and things invisible, whether thrones or dominions or principalities or powers; all things have been created through Him and in Him, and He is before all, and for Him all things consist. 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 through Him all things should be reconciled to Him</em>. So through the power of these works He is the image of God. For assuredly the Creator of things invisible is not compelled by any necessity inherent in His nature to be the visible image of the invisible God. And lest He should be regarded as the likeness of the form and not of the nature, He is styled the likeness of the invisible God in order that we may understand by His exercise of the powers (not the invisible attributes) of the Divine nature, that that nature is in Him.
Source: On the Trinity (New Advent)