By her very Mission, ‘The Church —– travels the same journey as all humanity and shares the same earthly lot with the world: she is to be a leaven and, as it were, the soul of human society in its renewal by Christ and transformation into the family of God’. Missionary endeavour requires patience. It begins with proclamation of the gospel to peoples and groups who do not yet believe in Christ, continues with the establishment of Christian Communities, that are a sign of God’s presence in the world, and leads to the foundation of local churches. It must involve a process of inculturation if the Gospel is to take flesh in each people’s culture. There will be times of defeat. ‘With regard to individuals, common groups and peoples it only by degrees that the Church touches and penetrates them, and so receives them into a fullness which is Catholic.
The Church’s Mission stimulates efforts towards Christian Unity. Indeed, ‘divisions among Christians prevent the Church from realising in practise the fullness of catholicity proper to her in those of her sons who, though joined to her by Baptism, are yet separated from full communion with her. Furthermore,the Church herself finds it more difficult to express in actual life her full catholicity in all its aspects.’
The missionary task implies a respectful dialogue with those who do not yet accept the Gospel. Believer can profit from this dialogue by learning to appreciate better “those elements of truth and grace which are found among peoples, and which are, as it were a secret presence of God”. They proclaim The Good News to those who do not it in order to consolidate, complete and raise the truth and the goodness that God has distributed among men and nations, and to purify them from error and evil “for the glory of God, the confusion of the demon and the happiness of man”
from the Catechism of the Catholic Church, nn. 854-856