1 It was the desire of Jesus Christ once He had wrought the Redemption of the human race by His death on the altar of the Cross, to lead men to obey His commands and thus win eternal life. To attain this end He used no other means than the voice of His heralds whose work it was to announce to all mankind what they had to believe and do in order to be saved. "It pleased God, by the foolishness of our preaching, to save them that believed." [I Cor . i:21] He chose therefore His Apostles, and after infusing into their minds by the power of the Holy Ghost, the gifts in harmony with their high calling, "Go ye into the world," He told them, "and preach the Gospel." [ Mark xvi, 15] Their preaching renewed the face of the earth. For if the religion of Christ has withdrawn the minds of men from errors of every kind to the truth, and won their hearts from the degradation of vice to the excellence and beauty of every virtue, assuredly it has done so by means of that very preaching. "Faith then cometh by hearing; and hearing by the word of Christ." [ Rom . x:17] Wherefore since by God's good pleasure, things are preserved through the same causes by which they were brought into being, it is evident that the preaching of the wisdom taught us by the Christian religion is the means Divinely employed to continue the work of eternal salvation, and that it must with just reason be looked upon as a matter of the greatest and most momentous concern. That preaching, therefore, must form the object of Our special care and attention, particularly so, if in any way, it may have lost perhaps some of its original perfection or its efficacy may have been impaired.
Source: Humani Generis Redemptionem (Vatican.va)