In his treatise Against Eunomius, Gregory begins with a discussion on “pure being,” which is, by necessity of its pure being-ness, infinite. All else that “be’s” is only able to do so as it moves towards that which is pure being. That which is totally pure is also infinite, for it lacks all detractive motion. The Father, Son, and Holy Spirit, since they share the divine being, are all equally infinite and incapable of admitting gradations among themselves. Gregory explains:
Good, as long as it is incapable of its opposite, has no bounds to its goodness: its opposite alone can circumscribe it, as we may see by particular examples. Strength is stopped only when weakness seizes it; life is limited by death alone; darkness is the ending of light: in a word, every good is checked by its opposite, and by that alone. If then he supposes that the nature of the Only-begotten and of the Spirit can change for the worse, then he plainly diminishes the conception of their goodness, making them capable of being associated with their opposites. But if the Divine and unalterable nature is incapable of degeneracy, as even our foes allow, we must regard it as absolutely unlimited in its goodness: and the unlimited is the same as the infinite. But to suppose excess and defect in the infinite and unlimited is to the last degree unreasonable: for how can the idea of infinitude remain, if we posited increase and loss in it? We get the idea of excess only by a comparison of limits: where there is no limit, we cannot think of any excess.
~Against Eunomius 1.15
Since there cannot be excess in pure being, it must be the case that the Son and Holy Spirit are also infinite, without any greater or lesser. To make them “less” is to put them on the other side of the infinite/finite divide, and thus to steal away their divine nature.