78 In the ecumenical movement, it is not only the Catholic Church and the Orthodox Churches which hold to this demanding concept of the unity willed by God. The orientation towards such unity is also expressed by others. Ecumenism implies that the Christian communities should help one another so that there may be truly present in them the full content and all the requirements of "the heritage handed down by the Apostles". Without this, full communion will never be possible. This mutual help in the search for truth is a sublime form of evangelical charity. The documents of the many International Mixed Commissions of dialogue have expressed this commitment to seeking unity. On the basis of a certain fundamental doctrinal unity, these texts discuss Baptism, Eucharist, ministry and authority. From this basic but partial unity it is now necessary to advance towards the visible unity which is required and sufficient and which is manifested in a real and concrete way, so that the Churches may truly become a sign of that full communion in the one, holy, catholic and apostolic Church which will be expressed in the common celebration of the Eucharist. This journey towards the necessary and sufficient visible unity, in the communion of the one Church willed by Christ, continues to require patient and courageous efforts. In this process, one must not impose any burden beyond that which is strictly necessary (cf. Acts 15:28).
Source: Ut Unum Sint (Vatican.va)