7 This missionary activity derives its reason from the will of God, "who wishes all men to be saved and to come to the knowledge of the truth. For there is one God, and one mediator between God and men, Himself a man, Jesus Christ, who gave Himself as a ransom for all" (1 Tim. 2:45), "neither is there salvation in any other" (Acts 4:12). Therefore, all must be converted to Him, made known by the Church's preaching, and all must be incorporated into Him by baptism and into the Church which is His body. For Christ Himself "by stressing in express language the necessity of faith and baptism (cf. Mark 16:16; John 3:5), at the same time confirmed the necessity of the Church, into which men enter by baptism, as by a door. Therefore those men cannot be saved, who though aware that God, through Jesus Christ founded the Church as something necessary, still do not wish to enter into it, or to persevere in it." Therefore though God in ways known to Himself can lead those inculpably ignorant of the Gospel to find that faith without which it is impossible to please Him (Heb. 11:6), yet a necessity lies upon the Church (1 Cor. 9:16), and at the same time a sacred duty, to preach the Gospel. And hence missionary activity today as always retains its power and necessity. By means of this activity, the Mystical Body of Christ unceasingly gathers and directs its forces toward its own growth (cf. Eph. 4:11-16). The members of the Church are impelled to carry on such missionary activity by reason of the love with which they love God and by which they desire to share with all men the spiritual goods of both its life and the life to come. Finally, by means of this missionary activity, God is fully glorified, provided that men fully and consciously accept His work of salvation, which He has accomplished in Christ. In this way and by this means, the plan of God is fulfilled - that plan to which Christ conformed with loving obedience for the glory of the Father who sent Him, that the whole human race might form one people of God and be built up into one temple of the Holy Spirit which, being the expression of brotherly harmony, corresponds with the inmost wishes of all men. And so at last, there will be realized the plan of our Creator who formed man to His own image and likeness, when all who share one human nature, regenerated in Christ through the Holy Spirit and beholding the glory of God, will be able to say with one accord: "Our Father."
Source: Ad Gentes (Vatican.va)