The Holy Spirit is that large river by which the mystical Jerusalem is watered. It is equal to its Fount, that is, the Father and the Son, as is signified in holy Scripture. St. Ambrose himself thirsts for that water, and warns us that in order to preserve it within us, we must avoid the devil, lust, and heresy, since our vessels are frail, and that broken cisterns must be forsaken, that after the example of the Samaritan woman and of the patriarchs we may find the water of the Lord.
176. But lest perchance any one should speak against as it were the littleness of the Spirit, and from this should endeavour to establish a difference in greatness, arguing that water seems to be but a small part of a Fount, although examples taken from creatures seem by no means suitable for application to the Godhead; yet lest they should judge anything injuriously from this comparison taken from creatures, let them learn that not only is the Holy Spirit called Water, but also a River, as we read: “From his belly shall flow rivers of living water. But this He said of the Spirit, Whom they were beginning to receive, who were about to believe in Him.”
177. 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.”
178. 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.
179. 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 fullness 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.
180. 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.
181. You observe, then, from His words that the unity of the divine greatness is pointed out, and that Christ cannot be denied to be a Fount even by heretics, since the Spirit, too, is called a Fount. And as the Spirit is called a river, so, too, the Father said: “Behold, I come down upon you like a river of peace, and like a stream overflowing the glory of the Gentiles.” And who can doubt that the Son of God is the River of life, from Whom the streams of eternal life flowed forth?
182. 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 your own vessels, and from the founts of your own wells, and let your waters flow abroad in your 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 You taught Your apostles, saying: “Lay not up for yourselves treasures upon the earth, where rust and moth destroy, and where thieves break through and steal.”
182. For He intimates that the thief is the unclean spirit, who cannot find entrance into those who walk in the light of good works, but if he has caught any one in the darkness of earthly desires, and in the midst of the enjoyment of earthly pleasures, he spoils them of all the flower of eternal virtue. And therefore the Lord says: “Lay up for yourselves treasures in heaven, where neither rust nor moth destroy, and where thieves do not break through and steal. For where your treasure is, there will your heart be also.”
183. Our rust is wantonness, our rust is lust, our rust is luxury, which dim the keen vision of the mind with the filth of vices. Again, our moth is Arius, our moth is Photinus, who rend the holy vesture of the Church with their impiety, and desiring to separate the indivisible unity of the divine power, gnaw the precious veil of faith with sacrilegious tooth. The water is spilt if Arius has imprinted his tooth, it flows away if Photinus has planted his sting in any one's vessel. We are but of common clay, we quickly feel vices. But no one says to the potter, “Why have You made me thus?” For though our vessel be but common, yet one is in honour, another in dishonour. Do not then lay open your pool, dig not with vices and crimes, lest any one say: “He has opened a pool and dug it, and is fallen into the pit which he made.”
184. If you seek Jesus, forsake the broken cisterns, for Christ was wont to sit not by a pool but by a well. There that Samaritan woman found Him, she who believed, she who wished to draw water. Although you ought to have come in early morning, nevertheless if you come later, even at the sixth hour, you will find Jesus wearied with His journey. He is weary, but it is through you, because He has long sought you, your unbelief has long wearied Him. Yet He is not offended if you only comest, He asks to drink Who is about to give. But He drinks not the water of a stream flowing by, but your salvation; He drinks your good dispositions, He drinks the cup, that is, the Passion which atoned for your sins, that you drinking of His sacred blood might quench the thirst of this world.
185. So Abraham gained God after he had dug the well. So Isaac, while walking by the well, received that wife who was coming to him as a type of the Church. Faithful he was at the well, unfaithful at the pool. Lastly, too, Rebecca, as we read, found him who sought her at the well, and the harlots washed themselves in the blood in the pool of Jezebel.
Source: On the Holy Spirit (New Advent)