The following is from Blessed Columba Marmion’s masterful Christ, the Life of the Soul. In the larger context, Abbot Marmion is considering the mystery of holiness, first in God, then in men. After speaking of holiness considered as an attribute of God (i.e., in the Divine Nature), he goes on to consider holiness in the Trinity:

Human reason can arrive at establishing the existence of this holiness in the Supreme Being, holiness which is an attribute, a perfection of the Divine nature considered in itself.

But Revelation brings us a new light.

Here we must reverently raise the eyes of our soul even to the sanctuary of the Adorable Trinity, we must hear what Jesus Christ, both to nourish our piety and to exercise our faith, has Himself willed to reveal to us, or to teach us through His Church, about the intimate life of God.

There are, as you know, Three Divine Persons in God, the Father, the Son, and the Holy Ghost, three distinct Persons, but all three having one and the same Nature or Divine Essence. Being infinite Intelligence, the Father perfectly knows His perfections, He expresses this knowledge in One Word, the living, substantial Word, the adequate expression of what the Father is. In uttering this Word, the Father begets the Son, to Whom He communicates all His Essence, His Nature, His Perfections, His Life: Sicut Pater habet vitam in semetipso, sic dedit et Filio habere vitam in semetipso [John 5:26: As the Father has live in himself, so also has he given the Son to have live in himself”]. The Son also belongs entirely to His Father, is entirely given up to Him by a total donation which pertains to His nature as Son. And from this mutual donation of one and the same love, proceeds, as from one principle, the Holy Spirit Who seals the union of the Father and the Son by being Their substantial and living Love.

This mutual communication of the three Persons, this infinite loving union between themselves assuredly constitutes a new revelation of holiness in God: it is the ineffable union of God with Himself in the unity of His Nature and the Trinity of Persons.

At this point in the text there is a footnote, and with this footnote we will conclude the Blessed Abbot’s thoughts. Here is where he answers the question Why is the Holy Ghost called “Holy”:

Let us here state, for the sake of those more initiated in theological questions that each Person of the Trinity is identical with the Divine Essence, and consequently is holy, with a substantial holiness, because each only acts conformably to this Essence considered as the supreme norm of life and activity. It may be added that the Persons are holy because each of Them gives Himself to and belongs to the Other in an act of infinite adhesion. Lastly, the Third Person is especially called holy, because He proceeds from the Two Others through love; love is the principal act by which the will tends towards its end and is united to it; it designates the most eminent act of adhesion to the norm of all goodness, that is to say, of holiness, and therefore, the Spirit, Who, in God, proceeds through love, bears pre-eminently the name of holy. This is the text of St. Thomas who exposes this beautiful and profound doctrine to us. Cum bonum amatum habeat rationem finis, ex fine autem ipsum bonum amatur, quod Deus est, eminentem quamdam obtineat bonitatem, QUAE NOMINE SANCTITATIS EXPRIMITUR… Igitur Spiritus quo nobis insinuatur amor quo Deus se amat, Spiritus Sanctus nominatur. Opiscula selecta, t. III, C. XLVII. It is to be seen from all this that we gain a more profound conception of Divine sanctity by considering the Trinity of Persons.

The Latin text of St. Thomas, which Abbot Marmion’s book leaves untranslated, is as follows. I am including the surrounding text as well:

“Since good that is loved has the nature of an end, and since the motion of the will is designated good or evil in terms of the end it pursues, the love whereby the supreme good that is God is loved must possess the supereminent goodness that goes by the name of holiness. This is true whether holy is taken as equivalent to ‘pure,’ according to the Greeks (the idea being that in God there is purest goodness free from all defect) or whether holy is taken to mean ‘firm,’ in the view of the Latins (on the score that in God there is unchangeable goodness). In either case, everything dedicated to God is called holy, such as a temple and the vessels of the temple and all objects consecrated to divine service. Rightly, then, the Spirit, who represents to us the love whereby God loves Himself, is called the Holy Spirit. For this reason the rule of the Catholic Faith proclaims that the Spirit is holy, in the clause, ‘I believe in the Holy Spirit.'”