While friendship with God is an important theme for Thomas Aquinas, Aquinas never explicitly delineates what such friendship looks like. In this article, I present a systematic description of friendship with God according to Thomas Aquinas. I examine three dynamics (mutuality, benevolence, and communicatio) and two effects (mutual indwelling and union) which Aquinas attributes to human friendship, and I show how they can exist analogically in friendship with God. Such a presentation reveals that friendship with God effects the deification of the human person, and such a deified condition is the intended state of the human person, in which he or she can be most fully human. This conclusion allows me to propose a way to define a human person as a relation (using the term deificatus), analogous to Aquinas's definition of person for the Trinity. In turn, I show how this reality impacts a Thomistic understanding of genuine human friendship, whereby one friend loves the other qua deificatus.