7 The manner and extent of the action of the Holy Ghost in individual souls is no less wonderful, although somewhat more difficult to understand, inasmuch as it is entirely invisible. This outpouring of the Spirit is so abundant, that Christ Himself, from whose gift it proceeds, compares it to an overflowing river, according to those words of St. John: "He that believeth in Me, as the Scripture saith, out of his midst shall flow rivers of livingwater"; to which testimony the Evangelist adds the explanation: "Nowthis He said of the Spirit which they should receive who believed in Him"(John vii., 38, 39). It is indeed true that in those of the just who livedbefore Christ, the Holy Ghost resided by grace, as we read in the Scripturesconcerning the prophets, Zachary, John the Baptist, Simeon, and Anna; so that onPentecost the Holy Ghost did not communicate Himself in such a way "as thenfor the first time to begin to dwell in the saints, but by pouring Himself forthmore abundantly; crowning, not beginning His gifts; not commencing a new work,but giving more abundantly" (St. Leo the Great, Hom. iii., de Pentec.). Butif they also were numbered among the children of God, they were in a state likethat of servants, for "as long as the heir is a child he differeth nothingfrom a servant, but is under tutors and governors" (Gal. iv., I, 2).Moreover, not only was their justice derived from the merits of Christ who wasto come, but the communication of the Holy Ghost after Christ was much moreabundant, just as the price surpasses in value the earnest and the realityexcels the image. Wherefore St. John declares: "As yet the Spirit was notgiven, because Jesus was not yet glorified" (John vii., 39). So soon,therefore, as Christ, "ascending on high," entered into possession ofthe glory of His Kingdom which He had won with so much labour, He munificentlyopened out the treasures of the Holy Ghost: "He gave gifts to men"(Eph. iv., 8). For "that giving or sending forth of the Holy Ghost afterChrist's glorification was to be such as had never been before; not that therehad been none before, but it had not been of the same kind" (St. Aug., DeTrin., 1. iv. c. 20).
Source: Divinum Illud Munus (Vatican.va)