56 Following the Second Vatican Council, and in the light of earlier tradition, it has again become usual to refer to the particular or local Churches gathered around their Bishop as "Sister Churches". In addition, the lifting of the mutual excommunications, by eliminating a painful canonical and psychological obstacle, was a very significant step on the way towards full communion. The structures of unity which existed before the separation are a heritage of experience that guides our common path towards the re-establishment of full communion. Obviously, during the second millennium the Lord has not ceased to bestow on his Church abundant fruits of grace and growth. Unfortunately, however, the gradual and mutual estrangement between the Churches of the West and the East deprived them of the benefits of mutual exchanges and cooperation. With the grace of God a great effort must be made to re-establish full communion among them, the source of such good for the Church of Christ. This effort calls for all our good will, humble prayer and a steadfast cooperation which never yields to discouragement. Saint Paul urges us: "Bear one another's burdens" ( Gal 6:2). How appropriate and relevant for us is the Apostle's exhortation! The traditional designation of "Sister Churches" should ever accompany us along this path.
Source: Ut Unum Sint (Vatican.va)