Thomas Aquinas

“ So no other effect can be put down as the reason why the divine person is in the rational creature in a new mode, except sanctifying grace. Hence, the divine person is sent, and proceeds temporally only according to sanctifying grace.
Again, we are said to possess only what we can freely use or enjoy: and to have the power of enjoying the divine person can only be according to sanctifying grace. And yet the Holy Ghost is possessed by man, and dwells within him, in the very gift itself of sanctifying grace. ”

“ In the sense of "giving" as a free bestowal of something, the Father gives Himself, as freely bestowing Himself to be enjoyed by the creature. But as implying the authority of the giver as regards what is given, "to be given" only applies in God to the Person Who is from another ”

“ For God is in all things by His essence, power and presence, according to His one common mode, as the cause existing in the effects which participate in His goodness. Above and beyond this common mode, however, there is one special mode belonging to the rational nature wherein God is said to be present as the object known is in the knower, and the beloved in the lover. ”

“ Thus the mission of a divine person is a fitting thing, as meaning in one way the procession of origin from the sender, and as meaning a new way of existing in another; thus the Son is said to be sent by the Father into the world, inasmuch as He began to exist visibly in the world by taking our nature ”

“ For that rock was already created, and after the manner of an action was named Christ, Whom it typified; whereas the dove and the fire suddenly appeared to signify only what was happening. They seem, however, to be like to the flame of the burning bush seen by Moses and to the column which the people followed in the desert, and to the lightning and thunder issuing forth when the law was given on the mountain. ”

“ Wherefore the invisible things of God must be made manifest to man by the things that are visible. As God, therefore, in a certain way has demonstrated Himself and His eternal processions to men by visible creatures, according to certain signs ”

“ To the apostles the mission was directed in the form of breathing to show forth the power of their ministry in the dispensation of the sacraments; and hence it was said, "Whose sins you shall forgive, they are forgiven" (Jn. 20:23) : and again under the sign of fiery tongues to show forth the office of teaching ”