6 Since Jesus Christ has proclaimed that the special sign of discipleship with Him is that we "have love one for another" ( John xiii, 35; xv, 12) can we give a mark of greater love for our neighbors than to assist them in putting behind themselves the darkness of error by instructing them in the true faith of Christ? As a matter of fact, this type of charity surpasses all other kinds of good works inspired by love just as the mind surpasses the body, heaven surpasses earth, eternity surpasses time. Every one that acts thus, inspired by love and according to the full measure of his ability, demonstrates that he esteems the gift of faith in the manner that one should esteem it. Moreover, he manifests his gratitude toward the goodness of God by thus sharing this same great gift, precious above every other gift, with the poor pagans. He also shares with them, at the same time, all the other graces which are intimately connected with the virtue of faith. If none of the faithful is exempt from the obligation of charity, can the clergy who, by their truly marvelous election and holy vocation, participate in the very priesthood and apostolate of Jesus Christ, claim such exemption? Or can you, Venerable Brothers, you who possess the plenitude of the priesthood and are, each in his own diocese, the divinely constituted pastors of the clergy and Christian people, claim to be exempt from the same law of love? We read that Christ commanded not only Peter, whose chair We occupy, but all the Apostles whose successors you are: "Go ye into the whole world, and preach the gospel to every creature." ( Mark xvi, 15) It evidently follows from this that the responsibility for spreading the faith falls upon Us, but on condition that you share with Us the burden and assist Us as much as your own pastoral duties permit. Therefore, Venerable Brothers, do not look upon compliance with this Our paternal exhortation, as an irksome duty, for you must know that God Himself shall one day ask of us a strict accounting of this tremendous obligation which He has laid upon Us.
Source: Rerum Ecclesiae (Vatican.va)