47 The Apostle then recognised that nothing can fall within our knowledge, except it be posterior in time to the faculty of sense. Accordingly when he had asserted the depth of the wisdom of God, the infinity of His inscrutable judgments, the secret of His unsearchable ways, the mystery of His unfathomable mind, the incomprehensibility of His uncommunicated counsel, he continued, For who has first given to Him, and it shall be recompensed unto him again? For of Him, and through Him, and in Him are all things.
The eternal God is neither subject to limitation, nor did human reason and intelligence exercise their functions before He had His being. His whole being is therefore a depth, which we can neither examine nor penetrate. We say His whole being, not to define it as limited, but to understand it in its unlimited boundlessness: because of no one has He received His being, no antecedent giver can claim service from Him in return for a gift bestowed: for of Him and through Him and in Him are all things.
He does not lack things that are of Him and through Him and in Him. The Source and Maker of all, Who contains all, Who is beyond all, does not need that which is within Him, the Creator His creatures, the Possessor His possessions. Nothing is prior to Him, nothing derived from any other than Him, nothing beyond Him. What element of fullness is still lacking in God, which time will supply to make Him all in all? Whence can He receive it, if outside Him is nothing, and while nothing is outside Him, He is eternally Himself?
And if He is eternally Himself, and there is nothing outside Him, with what increase shall He be made full, by what addition shall He be made other than He is? Did He not say, I am and I change not ? What possibility is there of change in Him? What scope for progress? What is prior to eternity? What more divine than God? The subjection of the Son will not therefore make God to be all in all, nor will any cause perfect Him, from Whom and through Whom and in Whom are all causes. He remains God as He ever was, and He needs nothing further, for what He is, He is eternally of Himself and for Himself.
Source: On the Trinity (New Advent)