75 For Christians, this cooperation, which draws its inspiration from the Gospel itself, is never mere humanitarian action. It has its reason for being in the Lord's words: "For I was hungry and you gave me food" ( Mt 25:35). As I have already emphasized, the cooperation among Christians clearly manifests that degree of communion which already exists among them. Before the world, united action in society on the part of Christians has the clear value of a joint witness to the name of the Lord. It is also a form of proclamation, since it reveals the face of Christ. The doctrinal disagreements which remain exercise a negative influence and even place limits on cooperation. Still, the communion of faith which already exists between Christians provides a solid foundation for their joint action not only in the social field but also in the religious sphere. Such cooperation will facilitate the quest for unity. The Decree on Ecumenism noted that "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".
Source: Ut Unum Sint (Vatican.va)