The Glory of God's Grace offers the first full-length comprehensive study of Thomas's teaching on deification in its scriptural, patristic, philosophical, developmental, and systematic context. Daria Spezzano traces Thomas's theology of deification throughout the Summa, exploring in depth how the notion of deification links his 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. Also examined are Thomas's other works, in particular his Scripture commentaries, as well as the evolution of his thought. Spezzano argues that Thomas's 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, accounting for both the primacy of divine causality in all its modes and the fullness of graced human freedom. The fruit of this theology is ultimately doxological: the deification of adopted sons gives praise to God's glory by fully manifesting God's gracious plan to share the divine life with rational creatures.