50 This solicitude will serve as a motivation and stimulus for a renewed commitment to ecumenism. The relationship between ecumenical activity and missionary activity makes it necessary to consider two closely associated factors. On the one hand, we must recognize that "the division among Christians damages the holy work of preaching the Gospel to every creature and is a barrier for many in their approach to the faith." The fact that the Good News of reconciliation is preached by Christians who are divided among themselves weakens their witness. It is thus urgent to work for the unity of Christians, so that missionary activity can be more effective. At the same time we must not forget that efforts toward unity are themselves a sign of the work of reconciliation which God is bringing about in our midst. On the other hand, it is true that some kind of communion, though imperfect, exists among all those who have received Baptism in Christ. On this basis the Council established the principle that "while all appearance of indifferentism and confusion is ruled out, as well as any appearance of unhealthy rivalry, Catholics should collaborate in a spirit of fellowship with their separated brothers and sisters in accordance with the norms of the Decree on Ecumenism: by a common profession of faith in God and in Jesus Christ before the nations - to the extent that this is possible - and by their cooperation in social and technical as well as in cultural and religious matters." Ecumenical activity and harmonious witness to Jesus Christ by Christians who belong to different churches and ecclesial communities has already borne abundant fruit. But it is ever more urgent that they work and bear witness together at this time when Christian and para-Christian sects are sowing confusion by their activity. The expansion of these sects represents a threat for the Catholic Church and for all the ecclesial communities with which she is engaged in dialogue. Wherever possible, and in the light of local circumstances, the response of Christians can itself be an ecumenical one.
Source: Redemptoris Missio (Vatican.va)