4 The People of God are joined together primarily by the word of the living God. And rightfully they expect this from their priests. Since no one can be saved who does not first believe, priests, as co-workers with their bishops, have the primary duty of proclaiming the Gospel of God to all. In this way they fulfill the command of the Lord: "Going therefore into the whole world preach the Gospel to every creature" (Mk 16:15), and they establish and build up the People of God. Through the saving word the spark of faith is lit in the hearts of unbelievers, and fed in the hearts of the faithful. This is the way that the congregation of faithful is started and grows, just as the Apostle describes: "Faith comes from hearing, and hearing through the word of Christ" (Rom 10:17). To all men, therefore, priests are debtors that the truth of the Gospel which they have may be given to others. And so, whether by entering into profitable dialogue they bring people to the worship of God, whether by openly preaching they proclaim the mystery of Christ, or whether in the light of Christ they treat contemporary problems, they are relying not on their own wisdom for it is the word of Christ they teach, and it is to conversion and holiness that they exhort all men. But priestly preaching is often very difficult in the circumstances of the modern world. In order that it might more effectively move men's minds, the word of God ought not to be explained in a general and abstract way, but rather by applying the lasting truth of the Gospel to the particular circumstances of life. The ministry of the word is carried out in many ways, according to the various needs of those who hear and the special gifts of those who preach. In areas or communities of non-Christians, the proclaiming of the Gospel draws men to faith and to the sacraments of salvation. In the Christian community, especially among those who seem to understand and believe little of what they practice, the preaching of the word is needed for the very ministering of the sacraments. They are precisely sacraments of faith, a faith which is born of and nourished by the word. This is especially true of the Liturgy of the Word in the celebration of Mass, in which the proclaiming of the death and resurrection of Christ is inseparably joined to the response of the people who hear, and to the very offering whereby Christ ratified the New Testament in his blood. In this offering the faithful are united both by their dispositions and by their discernment of the sacrament.
Source: Presbyterorum Ordinis (Vatican.va)