Thomas Aquinas, Compendium of Theology

Another point to consider is this. 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 most pure 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 Catholic faith proclaims that the Spirit is holy, in the clause, "I believe in the Holy Ghost."