1 In reviewing attentively the history of the Church, one cannot fail to notice how, from the first ages of Christianity, the especial care and solicitude of the Roman Pontiffs have been directed to the end that they, undeterred by difficulties and obstacles, might spread the light of the Gospel and the benefits of Christian culture and civilization to the peoples who "sat in darkness and in the shadow of death." The Church has no other reason for existence than, by developing the Kingdom of Christ on earth, to make mankind participate in the effects of His saving Redemption. Whoever, by Divine Commission, takes the place on earth of Jesus Christ, becomes thereby the Chief Shepherd who, far from being able to rest content with simply guiding and protecting the Lord's Flock which has beer; confided to him to rule, fails in his special duty and obligations if he does not strive by might and main to win over and to join to Christ all who are still without the Fold.
Source: Rerum Ecclesiae (Vatican.va)