God is an intense and eternal lover. It was because of his love that he created you, and it is because of his love that he wishes to be united with you, to be present in you in a very special way. It is most important for us to realize that God's grace is more than merely some created thing that God imprints on us as a sign of his ownership -- it is not just a brand mark or a tag of ownership signed "God." The full meaning of sanctifying grace is that God himself, that is, the Holy Trinity -- Father, Son and Holy Spirit, is personally present in me in a way that he is not present in the rest of the material universe.
The Bible says that God, through grace, dwells in me, makes his home in me. For example, Jesus says in John 14:23, "If anyone loves me he will keep my word, and my Father will love him, and we shall come to him and make our home with him."
When the three divine Persons come to the sanctified believer, they come to him according to the special characteristics of their origin and procession. These are the truths about the Trinity that I have already explained. The NT uses the word "sending" or mission in this regard. A divine mission in this context is the procession of one Person from another with reference to a new way of existing in an external term. The point is that, because of the divine processions, the Father sends the Son, and the Father and the Son send the Holy Spirit. Thus, St. Paul says in Galatians 4:4, "God sent his Son." Both the Father and the Son are described as sending the Holy Spirit: "But the Advocate, the Holy Spirit, whom the Father will send in my name, will teach you everything and remind you of all I have said to you" (John 14:26). In the following chapter of John, Jesus says that he will send the Spirit: "When the Advocate comes, whom I shall send to you from the Father...he will be my witness" (15:26).
The Blessed Trinity is the source or cause of all creation. The final end or purpose of the universe is also the Trinity. By reason of his creative power, God is present in all creation, sustaining all things in existence. Irrational creatures, that is, all things beneath man, glorify God necessarily by their very existence. The glory of God from rational creatures requires free worship, praise and honor. Through the sin of our first parents, Adam and Eve, we lost the grace that God had intended for us to have. That situation was abundantly restored by the Incarnation of the second Person of the Blessed Trinity in Jesus Christ. God's will is that through faith in Jesus Christ, the acceptance of Baptism and incorporation into his Church, we should be made temples of the Holy Spirit, children of God and heirs of heaven. This is all accomplished through the indwelling of the Holy Trinity in the souls of the just.
The "indwelling" means that Father, Son and Holy Spirit become personally present to us through grace in a unique way. Their presence in the soul affects both our knowledge and our love. When the Bible speaks about "sending" or "mission" of the Son and the Holy Spirit it is referring to the special way in which they proceed in the Trinity itself. The temporal missions, therefore, reflect the individual characteristics of the divine Persons: The Father sends, but is not sent; the Son is sent and sends; the Holy Spirit is sent, but does not send.
In the course of salvation history, or God's dealing with mankind, we discover both external and internal divine missions. They could also be called visible and invisible missions. Thus, the Word of God became man in Jesus of Nazareth. That is what is meant by a visible mission. The Holy Spirit appeared under the form of a dove at the Baptism of Jesus (Matt. 3:16), under the form of a brilliant cloud at the Transfiguration of Jesus on Mount Tabor (Matt. 17:5), and under the form of tongues of fire in the upper room on Pentecost (Acts 2:3-4).
The visible missions are external signs of the invisible missions, namely, the Trinity dwelling in the souls of the just. This means that the Son and the Holy Spirit are present in a new manner in creatures. It is a new, interior and invisible presence, which sanctifies the soul and imparts to it a new supernatural life. The Father is also present in the sanctified soul because, as we have seen, where the Son and the Spirit are present, the Father, who is one with them, is present also. So when we pray to God within us we should pray to the Father, through the Son, in the Holy Spirit.
The sending of the Son and the Holy Spirit is expressed in the following texts: John 5:23; 6:38-39; 7:28;14:26; 15:26; 16:7; Galatians 4:4 and 6.
The indwelling in the soul of the just is common to the three Persons. We know, because consubstantiality and circumincession, that one Person cannot dwell in the soul without the others. Therefore, the indwelling is necessarily common and so the formal reason for the indwelling cannot be something personal, but it must be something substantial, namely, participation in the divine nature.
What is proper to the individual Persons in God is only what is of a relative nature or hypostatic nature. But indwelling or union with the soul of the just, however it is understood, cannot pertain to the relative order ad intra nor to the order of a hypostatic union. Therefore there cannot be found in the Holy Spirit any special reason because of which he is communicated to the just, and which would be the reason why the other Persons are communicated. In this there is a difference with the Incarnation which pertains to the Hypostasis as such. Therefore, the three persons cannot inhabit or be united to the soul of the just because of an individual or singular title, but only a common title.
This consideration brings us back to the notion of appropriation, which means attributing to one Person something that is common to all three. Appropriation is its positive foundation in the frequent and emphatic attribution in the Bible to one Person what is also said of the others. The indwelling in the just, both in Scripture and in the Fathers, is affirmed of the three Persons but frequently it is attributed to the Holy Spirit.
At the end of this course you should now be able to penetrate more deeply into the meaning of two key texts about the Trinity:
2 Cor. 13:13, The grace of the Lord Jesus Christ and the love of God and the fellowship of the Holy Spirit be with you all.
Matt. 28:19, Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit.
St. Thomas Aquinas, Summa Theologica I, Q. 42,
a. 5; Q. 43, aa. 1-8.