Who can listen to the Spirit’s titles and not be lifted up in his soul? Whose thoughts would not be raised to contemplate the supreme nature? He is called the Spirit of God, the Spirit of truth who proceeds from the Father, right Spirit, willing Spirit. His first and most proper title is Holy Spirit, a name most especially appropriate to everything which is incorporeal, purely immaterial, and indivisible. That is why the Lord taught the Samaritan woman, who thought that God had to be worshipped in specific places, that “God is Spirit.”
He wanted to show that an incorporeal being cannot be limited. When we hear the word “spirit” it is impossible for us to conceive of something whose nature can be limited or is subject to change or variation, or is like a creature in any way. Instead, we are compelled to direct our thoughts on high, and to think of an intelligent being, boundless in power, of unlimited greatness, generous in goodness, whom time cannot measure.
All things thirsting for holiness turn to Him; everything living in virtue never turns away from Him. He waters them with His life-giving breath and helps them reach their proper fulfillment. He perfects all other things, and Himself lacks nothing; He gives life to all things, and is never depleted. He does not increase by additions, but is always complete, self-established, and present everywhere. He is the source of sanctification, spiritual light, who gives illumination to everyone using His powers to search for the truth—and the illumination He gives us is Himself.
His nature is unapproachable; only through His goodness are we able to draw near it. He fills all things with His power, but only those who are worthy may share it. He distributes His energy in proportion to the faith of the recipient, not confining it to a single share. He is simple in being; His powers are manifold: they are wholly present everywhere and in everything. He is distributed but does not change.
He is shared, yet remains whole. Consider the analogy of the sunbeam: each person upon whom its kindly light falls rejoices as if the sun existed for him alone, yet it illumines land and sea, and is master of the atmosphere. In the same way, the Spirit is given to each one who receives Him as if he were the possession of that person alone, yet he sends forth sufficient grace to fill the entire universe. Everything that partakes of His grace is filled with joy according to its capacity—the capacity of its nature, not of His power.
From: On the Holy Spirit