39. Furthermore, given the very nature of ecclesial communion and its relation to the sacrament of the Eucharist, it must be recalled that "the Eucharistic Sacrifice, while always offered in a particular community, is never a celebration of that community alone. In fact, the community, in receiving the Eucharistic presence of the Lord, receives the entire gift of salvation and shows, even in its lasting visible particular form, that it is the image and true presence of the one, holy, catholic and apostolic Church." . . .
The ecclesial communion of the Eucharistic assembly is a communion with its own Bishop and with the Roman Pontiff. The Bishop, in effect, is the visible principle and the foundation of unity within his particular Church. It would therefore be a great contradiction if the sacrament par excellence of the Church's unity were celebrated without true communion with the Bishop. As Saint Ignatius of Antioch wrote: "That Eucharist which is celebrated under the Bishop, or under one to whom the Bishop has given this charge, may be considered certain." Likewise, since "the Roman Pontiff, as the successor of Peter, is the perpetual and visible source and foundation of the unity of the Bishops and of the multitude of the faithful," communion with him is intrinsically required for the celebration of the Eucharistic Sacrifice. Hence the great truth expressed which the Liturgy expresses in a variety of ways: "Every celebration of the Eucharist is performed in union not only with the proper Bishop, but also with the Pope, with the episcopal order, with all the clergy, and with the entire people. Every valid celebration of the Eucharist expresses this universal communion with Peter and with the whole Church, or objectively calls for it, as in the case of the Christian Churches separated from Rome. . . . "
43. In considering the Eucharist as the sacrament of ecclesial communion, there is one subject which, due to its importance, must not be overlooked: I am referring to the relationship of the Eucharist to ecumenical activity. We should all give thanks to the Blessed Trinity for the many members of the faithful throughout the world who in recent decades have felt an ardent desire for unity among all Christians. The Second Vatican Council, at the beginning of its Decree on Ecumenism, sees this as a special gift of God. It was an efficacious grace which inspired us, the sons and daughters of the Catholic Church and our brothers and sisters from other Churches and Ecclesial Communities, to set forth on the path of ecumenism.
Our longing for the goal of unity prompts us to turn to the Eucharist, which is the supreme sacrament of the unity of the People of God, in as much as it is the apt expression and the unsurpassable source of that unity. In the celebration of the Eucharistic Sacrifice the Church prays that God, the Father of mercies, will grant his children the fullness of the Holy Spirit so that they may become one body and one spirit in Christ. . . .
44. Precisely because the Church's unity, which the Eucharist brings about through the Lord's sacrifice and by communion in his body and blood, absolutely requires full communion in the bonds of the profession of faith, the sacraments and ecclesiastical governance, it is not possible to celebrate together the same Eucharistic liturgy until those bonds are fully re-established. Any such concelebration would not be a valid means, and might well prove instead to be an obstacle, to the attainment of full communion, by weakening the sense of how far we remain from this goal and by introducing or exacerbating ambiguities with regard to one or another truth of the faith. The path towards full unity can only be undertaken in truth. In this area, the prohibitions of Church law leave no room for uncertainty, in fidelity to the moral norm laid down by the Second Vatican Council.