16 Already from the earliest times the Eastern Churches followed their own forms of ecclesiastical law and custom, which were sanctioned by the approval of the Fathers of the Church, of synods, and even of ecumenical councils. Far from being an obstacle to the Church's unity, a certain diversity of customs and observances only adds to her splendor, and is of great help in carrying out her mission, as has already been stated. To remove, then, all shadow of doubt, this holy Council solemnly declares that the Churches of the East, while remembering the necessary unity of the whole Church, have the power to govern themselves according to the disciplines proper to them, since these are better suited to the character of their faithful, and more for the good of their souls. The perfect observance of this traditional principle not always indeed carried out in practice, is one of the essential prerequisites for any restoration of unity.
Source: Unitatis Redintegratio (Vatican.va)