This dissertation examines Thomas Aquinas’ theology of sanctification as deification in the Summa theologiae, through a contextualized analysis of his treatments of grace, charity and the gift of wisdom in the imago Dei on the journey to beatitude. The Summa represents Thomas’ mature thought on the graced movement of the elect towards God" conformed to, and moved by, their Trinitarian exemplar. Here, Thomas defines grace as a created habitus which is a deifying participation in the divine nature, and charity and wisdom as participations in the likeness of the Holy Spirit and Son. Deification is thus a true conformation to the image of the Trinity, and to the image of the Son, and at the same time, takes place through the graced creature’s own free moral activities, led by the Holy Spirit.
Chapters One and Two examine God’s participation of the divine perfections to creatures, and especially to the imago Dei by nature, grace and glory. Chapter Three treats participation in the divine nature by grace, as the principle of supernatural activities; Chapter Four, Christ as source and model of grace and adopted sonship. Chapters Five and Six examine charity and wisdom, which deify the will and intellect, ordering adopted children to beatitude as participants in Christ’s fellowship with the Father, especially through his sacraments. Attention is given to Thomas’ use of Scripture and developments from earlier works. In Chapter Seven, a theology of deification is traced throughout the Summa, linking treatments of the divine missions and image, the journey to beatitude through the moral life, adopted sonship through Christ and his sacraments, and the deiform worship of the beatific vision. Thomas’ theology of deification in the Summa theologiae demonstrates his mature vision of God’s loving and sapiential ordering of predestined human persons to communion with himself, by a progressive participation in the divine likeness and activity, which accounts for both the primacy of divine causality in all its modes, and the fullness of graced human freedom. The deification of adopted sons reveals God’s glory, by fully manifesting God’s gracious plan to share the divine life with rational creatures.