So, then, the Holy Spirit is the River, and the abundant River, which according to the Hebrews flowed from Jesus in the lands, as we have received it prophesied by the mouth of Isaiah. This is the great River which flows always and never fails. And not only a river, but also one of copious stream and overflowing greatness, as also David said: "The stream of the river makes glad the city of God."
For neither is that city, the heavenly Jerusalem, watered by the channel of any earthly river, but that Holy Spirit, proceeding from the Fount of Life, by a short draught of Whom we are satiated, seems to flow more abundantly among those celestial Thrones, Dominions and Powers, Angels and Archangels, rushing in the full course of the seven virtues of the Spirit. For if a river rising above its banks overflows, how much more does the Spirit, rising above every creature, when He touches the as it were low-lying fields of our minds, make glad that heavenly nature of the creatures with the larger fertility of His sanctification.
And let it not trouble you that either here it is said "rivers," or elsewhere "seven Spirits," for by the sanctification of these seven gifts of the Spirit, as Isaiah said, is signified the fulness of all virtue; the Spirit of wisdom and understanding, the Spirit of counsel and strength, the Spirit of knowledge and godliness, and the Spirit of the fear of God. One, then, is the River, but many the channels of the gifts of the Spirit. This River, then, goes forth from the Fount of Life.
And here, again, you must not turn aside your thoughts to lower things, because there seems to be some difference between a Fount and a River, and yet the divine Scripture has provided that the weakness of human understanding should not be injured by the lowliness of the language. Set before yourself any river, it springs from its fount, but is of one nature, of one brightness and beauty. And do you assert rightly that the Holy Spirit is of one substance, brightness, and glory with the Son of God and with God the Father. I will sum up all in the oneness of the qualities, and shall not be afraid of any question as to difference of greatness. For in this point also Scripture has provided for us; for the Son of God says: "He that shall drink of the water which I will give him, it shall become in him a well of water springing up unto everlasting life." This well is clearly the grace of the Spirit, a stream proceeding from the living Fount. The Holy Spirit, then, is also the Fount of eternal life. . . .
Good, then, is this water, even the grace of the Spirit. Who will give this Fount to my breast? Let it spring up in me, let that which gives eternal life flow upon me. Let that Fount overflow upon us, and not flow away. For Wisdom says: "Drink water out of thine own vessels, and from the founts of thine own wells, and let thy waters flow abroad in thy streets." How shall I keep this water that it flow not forth, that it glide not away? How shall I preserve my vessel, lest any crack of sin penetrating it, should let the water of eternal life exude? Teach us, Lord Jesus, teach us as Thou didst teach Thine apostles, saying: "Lay not up for yourselves treasures upon the earth, where rust and moth destroy, and where thieves break through and steal."
On the Holy Spirit, I.177-82