10 These sublime truths, which so clearly show forth the infinite goodness of the Holy Ghost towards us, certainly demand that we should direct towards Him the highest homage of our love and devotion. Christians may do this most effectually if they will daily strive to know Him, to love Him, and to implore Him more earnestly; for which reason may this Our exhortation, flowing spontaneously from a paternal heart, reach their ears. Perchance there are still to be found among them, even nowadays, some, who if asked, as were those of old bySt. Paul the Apostle, whether they have received the Holy Ghost, might answer inlike manner: "We have not so much as heard whether there be a HolyGhost" (Acts xix., 2). At least there are certainly many who are verydeficient in their religious practices, but their faith is involved in muchdarkness. Wherefore all preachers and those having care of souls should rememberthat it is their duty to instruct their people more diligently and more fullyabout the Holy Ghost-avoiding, however, difficult and subtle controversies, andeschewing the dangerous folly of those who rashly endeavour to pry into divinemysteries. What should be chiefly dwelt upon and clearly explained is themultitude and greatness of the benefits which have been bestowed, and areconstantly bestowed, upon us by this Divine Giver, so that errors and ignoranceconcerning matters of such moment may be entirely dispelled, as unworthy of"the children of light." We urge this, not only because it affects amystery by which we are directly guided to eternal life, and which musttherefore be firmly believed; but also because the more clearly and fully thegood is known the more earnestly it is loved. Now we owe to the Holy Ghost, aswe mentioned in the second place, love, because He is God: "Thou shalt lovethe Lord thy God with thy whole heart, and with thy whole soul, and with thywhole strength" (Deut. vi., 5). He is also to be loved because He is thesubstantial, eternal, primal Love, and nothing is more lovable than love. Andthis all the more because He has overwhelmed us with the greatest benefits,which both testify to the benevolence of the Giver and claim the gratitude ofthe receiver. This love has a twofold and most conspicuous utility. In the firstplace it will excite us to acquire daily a clearer knowledge about the HolyGhost; for, as the Angelic Doctor says, "the lover is not content with thesuperficial knowledge of the beloved, but striveth to inquire intimately intoall that appertains to the beloved, and thus to penetrate into the interior; asis said of the Holy Ghost, Who is the Love of God, that He searcheth even theprofound things of God" (1 Cor. ii., 10; Summ. Theol., la. 2ae., q. 28, a.2). In the second place it will obtain for us a still more abundant supply ofheavenly gifts; for whilst a narrow heart contracteth the hand of the giver, a grateful and mindful heart causeth it to expand. Yet we must strive that this love should be of such a nature as not to consist merely in dry speculations or external observances, but rather to run forward towards action, and especially to fly from sin, which is in a more special manner offensive to the Holy Spirit. For whatever we are, that we are by the divine goodness; and this goodness is specially attributed to the Holy Ghost. The sinner offends this his Benefactor, abusing His gifts; and taking advantage of His goodness becomes more hardened in sin day by day. Again, since He is the Spirit of Truth, whosoever faileth by weakness or ignorance may perhaps have some excuse before Almighty God; but he who resists the truth through malice and turns away from it, sins most grievously against the Holy Ghost. In our days this sin has become so frequent that those dark times seem to have come which were foretold by St. Paul, in which men, blinded by the just judgment of God, should take falsehood for truth, and should believe in "the prince of this world," who is a liar and the father thereof, as a teacher of truth: "God shall send them the operation of error, to believe lying (2 Thess. ii., 10). In the last times some shall depart from the faith, giving heed to spirits of error and the doctrines of devils" (1 Tim. iv., 1). But since the Holy Ghost, as We have said, dwells in us as in His temple, We must repeat the warning of the Apostle: "Grieve not the Holy Spirit of God, whereby you are sealed" (Eph. iv., 30). Nor is it enough to fly from sin; every Christian ought to shine with the splendour of virtue so as to be pleasing to so great and so beneficent a guest; and first of all with chastity and holiness, for chaste and holy things befit the temple. Hence the words of the Apostle: "Know you not that you are the temple of God, and that the Spirit of God dwelleth in you? But if any man violate the temple of God, him shall God destroy. For the temple of God is holy, which you are" (1 Cor. iii., 16-17): a terrible, indeed, but a just warning.
Source: Divinum Illud Munus (Vatican.va)