Interpreting and understanding the writings of St. Thomas Aquinas requires knowing how he structures his texts. In the Summa Theologica, he gives an account of all of theology in an ordered manner. This order reveals that St. Thomas had an organic view of the mysteries of the faith in which everything was interconnected.
In the Summa, St. Thomas follows an exitus-reditus pattern. He begins with the one God; then discusses the procession of the persons of the Trinity followed by the processions of creatures. He then considers how creatures image God, and ultimately how they return back to him by intensifying that image through growth in virtue assisted by grace. Today's Scripture readings concern this feature.
"Everything that the Father has is mine." In the Creed, we call this reality consubstantial. The Son has everything that the Father has: the same power, goodness, wisdom, and limitlessness. It's not just that they have equal power as if there were two, separate powers, one possessed by the Son and the other by the Father, but that they share the same exact power which is the principle of all their actions. The Spirit is sent by the Father and the Son, and we believe that this gives us a glimpse of what the inner life of the Trinity is like. Because the Spirit is sent by the Father and Son, He proceeds from the Father and the Son.
Creatures also proceed from God. However, while creatures only imperfectly resemble God, the Son and the Spirit perfectly resemble the Father. There is no greater or less than in God, for the Father and the Son and the Spirit all share the same power. God created us, His creatures, solely because of love. St. Paul writes, "The God who made the world and all that is in it, the Lord of heaven and earth, does not dwell in sanctuaries made by human hands, nor is he served by human hands because he needs anything. God did not create us because he needed us in any capacity; rather he created us for His own sake.
God created us and now gives life to everyone among the whole human race. He remains close to us, so that we may find him. In God we have our being and movement. He is always guiding us with his providence. In a way similar to His gift to His Son, the Father also shares with us "because he will take from what is [Christ's] and declare it to you."
This is what we call grace: it is a share in the life of the blest Trinity. What once belonged only to the Son has been made available to us by the sending of the Holy Spirit, who guides us into all truth. By grace, we become more perfect images of the Father, lacking in nothing. We more perfectly imitate the Son's procession from the Father by following Christ's command to "be perfect as your heavenly father is perfect." The Father creates us, and we return to Him by being more closely united to his Son. By this mysterious working of the Holy Spirit, everything that both the Son and the Father have becomes available to us.
So, we can see St. Thomas's exitus-reditus pattern. From the Father processes the persons of the Trinity. From the Triune God processes all the creatures of the Earth. Rational creatures then return back to God by His grace which is the cause of our future glory in Heaven.