By St. Cyril of Jerusalem
"The water I shall give him will become in him a fountain of living water, welling up into eternal life" (Jn. 4:14). This is a new kind of water, a living, leaping water, welling up for those who are worthy. But why did Christ call the grace of the Spirit water? Because all things are dependent on water; plants and animals have their origin in water. Water comes down from heaven as rain, and although it is always the same in itself, it produces many different effects, one in the palm tree, another in the vine, and so on throughout the whole of creation. It does not come down, now as one thing, now as another, but while remaining essentially the same, it adapts itself to the needs of every creature that receives it.
In the same way the Holy Spirit, whose nature is always the same, simple and indivisible, apportions grace to each man as He wills. Like a dry tree which puts forth shoots when watered, the soul bears the fruit of holiness when repentance has made it worthy of receiving the Holy Spirit. Although the Spirit never changes, the effects of His action, by the will of God and in the name of Christ, are both many and marvelous.
The Spirit makes one man a teacher of divine truth, inspires another to prophesy, gives another the power of casting out devils, enables another to interpret Holy Scripture. The Spirit strengthens one man's self-control, shows another how to help the poor, teaches another to fast and lead a life of asceticism, makes another oblivious to the needs of the body, trains another for martyrdom.
His action is different in different people, but the Spirit Himself is always the same. In each person, Scripture says, the Spirit reveals His presence in a particular way for the common good.
The Spirit comes gently and makes himself known by His fragrance. He is not felt as a burden, for He is light, very light. Rays of light and knowledge stream before Him as He approaches. The Spirit comes with the tenderness of a true friend and protector to save, to heal, to teach, to counsel, to strengthen, to console. The Spirit comes to enlighten the nous first of the one who receives Him, and then, through Him, the nous of others as well.
As light strikes the eyes of a man who comes out of darkness into the sunshine and enables him to see clearly things he could not discern before, so light floods the soul of the man counted worthy of receiving the Holy Spirit and enables him to see things beyond the range of human vision, things hitherto undreamed of.
From "On the Holy Spirit".