1 Divinely sent to the nations of the world to be unto them "a universal sacrament of salvation," the Church, driven by the inner necessity of her own catholicity, and obeying the mandate of her Founder (cf. Mark 16:16), strives ever to proclaim the Gospel to all men. The Apostles themselves, on whom the Church was founded, following in the footsteps of Christ, "preached the word of truth and begot churches." It is the duty of their successors to make this task endure "so that the word of God may run and be glorified (2 Thess. 3:1) and the kingdom of God be proclaimed and established throughout the world. In the present state of affairs, out of which there is arising a new situation for mankind, the Church, being the salt of the earth and the light of the world (cf. Matt. 5:13-14), is more urgently called upon to save and renew every creature, that all things may be restored in Christ and all men may constitute one family in Him and one people of God. Therefore, this sacred synod, while rendering thanks to God for the excellent results that have been achieved through the whole Church's great - hearted endeavor, desires to sketch the principles of missionary activity and to rally the forces of all the faithful in order that the people of God, marching along the narrow way of the Cross, may spread everywhere the reign of Christ, Lord and overseer: of the ages (cf. Ecc. 36:19), and may prepare the way for his coming.
Source: Ad Gentes (Vatican.va)