St. Paul writes to his flock at Corinth: “Know you not that you are the temple of God, and that the Spirit of God dwelleth in you?” (1 Cor 3:16) And, to the Christians of Rome, he says: “The charity of God is poured forth in our hearts by the Holy Ghost, Who is given to us” (Rom 5:5).
The Jesuit Father Francis Cassilly comments: “From these and other passages, taken in connection with their context, the Fathers and theologians draw the conclusion that the Third Person of the Blessed Trinity dwells or abides within the soul of every just man. It is evident that the mysterious work of justification or sanctification is in some way to be attributed to the Holy Spirit. The soul is formally justified by grace, and this grace was merited for us by Christ in His passion, but it is poured forth in us by the Holy Ghost. The Spirit proceeds from the Father and the Son, and He is sent to us by Them to sanctify us, as the Son was sent by His Father to redeem us. Christ promised His Apostles that He would ask the Father, and He would give them another Paraclete, to abide with them forever (Jn 14:16).”
“But the work of the Holy Ghost is not done when He sanctifies us by pouring out charity in our heart. . . . When Love comes He cannot bear to leave; so He stays with us and by us. He takes up His abode within us, dwells with us, making our soul His home, from which He will never go out unless we cast Him forth by grievous sin.”
“God, having the attribute of immensity by which He is present everywhere, is said to exist in all creatures by His power, His presence and His essence. St. Thomas endeavors to illustrate this triple mode of existence, by saying that a king, since his authority extends to his whole kingdom, can be said to be present in every part of it by his power. A man is present to everything within his sight; and where his substance or being is he exists according to his substance or essence. (Summa Theologica I, 8, 3) Similarly, God is in all things by virtue of His power which reaches to every place and thing. He is actually present to all creatures, since all things are in His sight, plain and visible to Him; and finally He is in everything by His essence, which is the cause of their existence.”
Quotations from Francis Cassilly, A Story of Love, 2d ed. (St. Louis: B. Herder, 1917).