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.90 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.91 In raising this prayer to the Father of lights, from whom comes every good endowment and every perfect gift (cf. Jas 1:17), the Church believes that she will be heard, for she prays in union with Christ her Head and Spouse, who takes up this plea of his Bride and joins it to that of his own redemptive sacrifice.
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,92 in fidelity to the moral norm laid down by the Second Vatican Council.93
I would like nonetheless to reaffirm what I said in my Encyclical Letter Ut Unum Sint after having acknowledged the impossibility of Eucharistic sharing: “And yet we do have a burning desire to join in celebrating the one Eucharist of the Lord, and this desire itself is already a common prayer of praise, a single supplication. Together we speak to the Father and increasingly we do so 'with one heart'”.94