5 From the very beginning, the Lord Jesus "called to Himself those whom He wished; and He caused twelve of them to be with Him, and to be sent out preaching (Mark 3:13; cf. Matt. 10:1-42). Thus the Apostles were the first budding - forth of the New Israel, and at the same time the beginning of the sacred hierarchy. Then, when He had by His death and His resurrection completed once for all in Himself the mysteries of our salvation and the renewal of all things, the Lord, having now received all power in heaven and on earth (cf. Matt. 28 18), before He was taken up into heaven (cf. Acts 1:11), founded His Church as the sacrament of salvation and sent His Apostles into all the world just as He Himself had been sent by His Father (cf. John 20:21), commanding them: "Go, therefore, and make disciples of a nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit; teaching them to observe all that I have commanded you" (Matt. 28:19 ff.). "Go into the whole world, preach the Gospel to every creature. He who believes and is baptized shall be saved; but he who does not believe, shall be condemned" (Mark 16:15ff.). Whence the duty that lies on the Church of spreading the faith and the salvation of Christ, not only in virtue of the express command which was inherited from the Apostles by the order of bishops, assisted by the priests, together with the successor of Peter and supreme shepherd of the Church, but also in virtue of that life which flows from Christ into His members; "From Him the whole body, being closely joined and knit together through every joint of the system, according to the functioning in due measure of each single part, derives its increase to the building up of itself in love" (Eph. 4:16). The mission of the Church, therefore, is fulfilled by that activity which makes her, obeying the command of Christ and influenced by the grace and love of the Holy Spirit, fully present to all men or nations, in order that, by the example of her life and by her preaching, by the sacraments and other means of grace, she may lead them to the faith, the freedom and the peace of Christ; that thus there may lie open before them a firm and free road to full participation in the mystery of Christ. Since this mission goes on and in the course of history unfolds the mission of Christ Himself, who was sent to preach the Gospel to the poor, the Church, prompted by the Holy Spirit, must walk in the same path on which Christ walked: a path of poverty and obedience, of service and self - sacrifice to the death, from which death He came forth a victor by His resurrection. For thus did all the Apostles walk in hope, and by many trials and sufferings they filled up those things wanting to the Passion of Christ for His body which is the Church (cf. Col. 1:24). For often, the blood of Christians was like a seed.
Source: Ad Gentes (Vatican.va)