This study intends to present synthetically the principles which Vatican II established for the Catholic Church’s path toward ecumenism. Such a study today appears essential, because the ecumenism of the early years after the Council seemed to easy, too fast and superficial. The texts of the Council, though, marked a profound turning point and these should now be taken into more serious consideration. The notion of the Church’s unity is at the center of ecumenical doctrine, not in itself, but in relationship to other notions, such as «diversity» and «historical variations» . This is the very essence of being Catholic and Apostolic. «Catholic», means unity in diversity and this study articulates the progressive steps of the Council’s teaching: from «absoluteness» to « fullness»; thence to the «meaning of limits», finally from internal unity to the unity of the world (mission). «Apostolic» means unity and continuity in changing times, or rather Tradition in its successive incarnations (tradition in history). But all then meets in the notion of the Holy. «Ecumenical style » makes love the measure of all, the measure and promotion of the truth. The Council’s doctrine, therefore, is not finished, but calls for hope in the Holy Spirit and therefore, in the future (pp.75-94).