40 Relations between Christians are not aimed merely at mutual knowledge, common prayer and dialogue. They presuppose and from now on call for every possible form of practical cooperation at all levels: pastoral, cultural and social, as well as that of witnessing to the Gospel message. "Cooperation among all Christians vividly expresses that bond which already unites them, and it sets in clearer relief the features of Christ the Servant". This cooperation based on our common faith is not only filled with fraternal communion, but is a manifestation of Christ himself. Moreover, ecumenical cooperation is a true school of ecumenism, a dynamic road to unity. Unity of action leads to the full unity of faith: "Through such cooperation, all believers in Christ are able to learn easily how they can understand each other better and esteem each other more, and how the road to the unity of Christians may be made smooth". In the eyes of the world, cooperation among Christians becomes a form of common Christian witness and a means of evangelization which benefits all involved.
Source: Ut Unum Sint (Vatican.va)