Gregory's second main contribution is his spiritual theology. He is the first Christian theologian to argue for the infinity of God. [[Origen]], often called a major influence on Gregory, had explicitly argued that God is limited, an essential notion in Platonism, since to be limited is to be clearly defined and knowable. Gregory, however, argues that if God is limited he must be limited by something greater than himself; he is therefore without boundaries. The idea had already been developed by neoplatonic philosophers, especially Plotinus, another important influence on Gregory, but he is the first Christian to defend it, apart from some hints in the work of [[Irenaeus]].
Accordingly, Gregory argues that since God is infinite he cannot be comprehended. In contrast, Origen had spoken of the spiritual journey as a progression of increasing illumination, as the mystic studies [[Holy Scripture|Scripture]] and comes to learn more about God.