Summary of Chapter 30: The Divine Missions (from White’s book, The Trinity)

“Mission . . . includes the eternal procession, with the addition of a temporal effect.” (ST I, q. 43, a. 2, ad 3).

The missions of the Son and the Spirit just are the eternal processions, insofar as these processions are newly rendered present to spiritual creatures (angels and human beings) by a new economic or temporal effect of grace.
The grace in question allows us to grasp who God truly is eternally (granting us knowledge of the processions themselves) and allows us to partake of the very life of the Trinity, by participation in the life of God.
Indeed, this centrality is thoroughly scriptural in origin. The missions are a keystone that supports the whole novelty of the revelation of the New Testament and its salvific significance. The basis for this novelty is found in the new disclosure in time of God’s inner identity as Triune and the pouring forth of his own communion of life and charity into the world.

Invisible missions pertain particularly to the sending of the Son and Spirit into the spiritual faculties of angels and human beings, whereby they enjoy friendship with God by grace, particularly through the theological virtues of faith, hope, and charity. Although God is present everywhere already as the Creator of all that is, as he sustains everything in existence, He can be present to the rational creature is an additional way through sanctifying grace (of which the principal component are the theological virtues) to become the temple of God. ST I, q. 43, a. 3, ad 2: “Sanctifying grace disposes the soul to possess the divine person: and this is signified when it is said that the Holy Spirit is given according to the gift of grace.”
Visible missions pertain to the sensible manifestations of the Son and Spirit in visible history, particularly through the incarnation and life, death, and resurrection of the Son, and the visible sending of the Holy Spirit in tongues of fire at Pentecost.

The theology of the missions of the persons of the Holy Trinity has a central significance for the whole of Aquinas’s thought. Aquinas first takes up the topic of the Trinitarian missions in the Summa in Prima Pars, question 43, a location in the text that is important for a variety of reasons. First and foremost, at this juncture in the Summa Aquinas is in the process of transitioning from his treatment of God to that of the procession of creatures from God via creation. He purposefully locates his treatment of the missions, then, between his reflection on the Trinity and his reflection on creation coming forth from God. This location signals to us that for Aquinas, at base, the mystery of creation is intimately related to the mystery of divinization or sanctification by grace. God created spiritual creatures in order to reveal himself to them, and to dwell in them by grace through the invisible missions of the Son and the Spirit. The angels and human beings who are created in the image of God are such primarily because of the twin spiritual faculties of intellect and will, which are capax Dei, capable of God, and which will in fact be divinized or sanctified by being plunged, as it were, into the mystery of the eternal Word, who is the contemplative light of the Father, and into the love of the Holy Spirit, who is the spirated love of the Father and his Word.