15 Everyone also knows with what great love the Christians of the East celebrate the sacred liturgy, especially the eucharistic celebration, source of the Church's life and pledge of future glory, in which the faithful, united with their bishop, have access to God the Father through the Son, the Word made flesh, Who suffered and has been glorified, and so, in the outpouring of the Holy Spirit, they enter into communion with the most holy Trinity, being made "sharers of the divine nature". Hence, through the celebration of the Holy Eucharist in each of these churches, the Church of God is built up and grows in stature and through concelebration, their communion with one another is made manifest. In this liturgical worship, the Christians of the East pay high tribute, in beautiful hymns of praise, to Mary ever Virgin, whom the ecumenical Council of Ephesus solemnly proclaimed to be the holy Mother of God, so that Christ might be acknowledged as being truly Son of God and Son of Man, according to the Scriptures. Many also are the saints whose praise they sing, among them the Fathers of the universal Church. These Churches, although separated from us, possess true sacraments, above all by apostolic succession, the priesthood and the Eucharist, whereby they are linked with us in closest intimacy. Therefore some worship in common (communicatio in sacris), given suitable circumstances and the approval of Church authority, is not only possible but to be encouraged. Moreover, in the East are found the riches of those spiritual traditions which are given expression especially in monastic life. There from the glorious times of the holy Fathers, monastic spirituality flourished which, then later flowed over into the Western world, and there provided the source from which Latin monastic life took its rise and has drawn fresh vigor ever since. Catholics therefore are earnestly recommended to avail themselves of the spiritual riches of the Eastern Fathers which lift up the whole man to the contemplation of the divine. The very rich liturgical and spiritual heritage of the Eastern Churches should be known, venerated, preserved and cherished by all. They must recognize that this is of supreme importance for the faithful preservation of the fullness of Christian tradition, and for bringing about reconciliation between Eastern and Western Christians.
Source: Unitatis Redintegratio (Vatican.va)