Let us now investigate what are our common conceptions concerning the Spirit, as well those which have been gathered by us from Holy Scripture concerning It as those which we have received from the unwritten tradition of the Fathers. First of all we ask, who on hearing the titles of the Spirit is not lifted up in soul, who does not raise his conception to the supreme nature? It is called “Spirit of God,” “Spirit of truth which proceedeth from the Father,” “right Spirit,” “a leading Spirit.” Its proper and peculiar title is “Holy Spirit;” which is a name specially appropriate to everything that is incorporeal, purely immaterial, and indivisible. So our Lord, when teaching the woman who thought God to be an object of local worship that the incorporeal is incomprehensible, said “God is a spirit.” On our hearing, then, of a spirit, it is impossible to form the idea of a nature circumscribed, subject to change and variation, or at all like the creature. We are compelled to advance in our conceptions to the highest, and to think of an intelligent essence, in power infinite, in magnitude unlimited, unmeasured by times or ages, generous of Its good gifts, to whom turn all things needing sanctification, after whom reach all things that live in virtue, as being watered by Its inspiration and helped on toward their natural and proper end; perfecting all other things, but Itself in nothing lacking; living not as needing restoration, but as Supplier of life; not growing by additions; but straightway full, self-established, omnipresent, origin of sanctification, light perceptible to the mind, supplying, as it were, through Itself, illumination to every faculty in the search for truth; by nature unapproachable, apprehended by reason of goodness, filling all things with Its power, but communicated only to the worthy; not shared in one measure, but distributing Its energy according to “the proportion of faith;” in essence simple, in powers various, wholly present in each and being wholly everywhere; impassively divided, shared without loss of ceasing to be entire, after the likeness of the sunbeam, whose kindly light falls on him who enjoys it as though it shone for him alone, yet illumines land and sea and mingles with the air. So, too, is the Spirit to every one who receives it, as though given to him alone, and yet It sends forth grace sufficient and full for all mankind, and is enjoyed by all who share It, according to the capacity, not of Its power, but of their nature.
—de Spiritu Sancto, Chapter IX (my emphasis)