By St. Basil the Great
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 proceeds from the Father, [John 15:26] 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. [John 4:24] 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; [Romans 12:6] 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.
Now the Spirit is not brought into intimate association with the soul by local approximation. How indeed could there be a corporeal approach to the incorporeal? This association results from the withdrawal of the passions which, coming afterwards gradually on the soul from its friendship to the flesh, have alienated it from its close relationship with God. Only then after a man is purified from the shame whose stain he took through his wickedness, and has come back again to his natural beauty, and as it were cleaning the Royal Image and restoring its ancient form, only thus is it possible for him to draw near to the Paraclete. And He, like the sun, will by the aid of your purified eye show you in Himself the image of the invisible, and in the blessed spectacle of the image you shall behold the unspeakable beauty of the archetype. Through His aid hearts are lifted up, the weak are held by the hand, and they who are advancing are brought to perfection. Shining upon those that are cleansed from every spot, He makes them spiritual by fellowship with Himself. Just as when a sunbeam falls on bright and transparent bodies, they themselves become brilliant too, and shed forth a fresh brightness from themselves, so souls wherein the Spirit dwells, illuminated by the Spirit, themselves become spiritual, and send forth their grace to others. Hence comes foreknowledge of the future, understanding of mysteries, apprehension of what is hidden, distribution of good gifts, the heavenly citizenship, a place in the chorus of angels, joy without end, abiding in God, the being made like to God, and, highest of all, the being made God. Such, then, to instance a few out of many, are the conceptions concerning the Holy Spirit, which we have been taught to hold concerning His greatness, His dignity, and His operations, by the oracles of the Spirit themselves.
Source: From On the Holy Spirit, Ch. 9.