This book is an investigation into two basic concepts of ancient pagan and Christian thought, namely activity and participation. It shows how activity in Christian thought is connected with the topic of participation: for the lower levels of being to participate in the higher means to receive the divine activity into their own ontological constitution. It is mainly a discussion of some important Church Fathers. Against a background of Aristotelian and Neoplatonist philosophy, the book discusses Gregory of Nyssa, Dionysius the Areopagite, Maximus the Confessor, and culminates with a chapter on Gregory Palamas before some conclusions are drawn. The concern of the author is to highlight how the Christians think energeia (i.e. activity or energy) is manifested as divine activity in the eternal constitution of the Trinity, the creation of the cosmos, the Incarnation of Christ, and in salvation understood as deification. Terms such as essence and energy are associated with the theology and spirituality of the fourteenth-century Byzantine thinker Gregory Palamas. One purpose of this book is to show how Palamas’ theology is in accordance with Greek patristic thinking, with its background in a definite trend in ancient pagan philosophy.