5 The Church which, already conceived, came forth from the side of the second Adam in His sleep on the Cross, first showed herself before the eyes of men on the great day of Pentecost. On that day the Holy Ghost began to manifest His gifts in the mystic body of Christ, by that miraculous outpouring already foreseen by the prophet Joel (ii., 28-29), for the Paraclete "sat upon the apostles as though new spiritual crowns were placed upon their heads in tongues of fire" (S. Cyril Hier. Catech. 17). Then the apostles "descended from the mountain," as St. John Chrysostom writes, "not bearing in their hands tables of stone like Moses, but carrying the Spirit in their mind, and pouring forth the treasure and the fountain of doctrines and graces" (In Matt. Hom. L, 2 Cor. iii., 3). Thus was fully accomplished that last promise of Christ to His apostles of sending the Holy Ghost, who was to complete and, as it were, to seal the deposit of doctrine committed to them under His inspiration. "I have yet many things to say to you, but you cannot bear them now; but when He, the Spirit of Truth, shall come, He will teach you all truth" (John xvi., 12-13). For He who is the Spirit of Truth, inasmuch as He proceedeth both from the Father, who is the eternally True, and from the Son, who is the substantial Truth, receiveth from each both His essence and the fullness of all truth. This truth He communicates to His Church, guarding her by His all powerful help from ever falling into error, and aiding her to foster daily more and more the germs of divine doctrineand to make them fruitful for the welfare of the peoples. And since the welfareof the peoples, for which the Church was established, absolutely requires thatthis office should be continued for all time, the Holy Ghost perpetuallysupplies life and strength to preserve and increase the Church. "I will askthe Father, and He will give you another Paraclete, that He may abide with youfor ever, the Spirit of Truth" (john xiv., 16, 17).
Source: Divinum Illud Munus (Vatican.va)