Note this part especially:

42. The safeguarding and promotion of ecclesial communion is a task of each member of the faithful, who finds in the Eucharist, as the sacrament of the Church’s unity, an area of special concern. More specifically, this task is the particular responsibility of the Church’s Pastors, each according to his rank and ecclesiastical office. For this reason the Church has drawn up norms aimed both at fostering the frequent and fruitful access of the faithful to the Eucharistic table and at determining the objective conditions under which communion may not be given. The care shown in promoting the faithful observance of these norms becomes a practical means of showing love for the Eucharist and for the Church.

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.89 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.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

45. While it is never legitimate to concelebrate in the absence of full communion, the same is not true with respect to the administration of the Eucharist under special circumstances, to individual persons belonging to Churches or Ecclesial Communities not in full communion with the Catholic Church. In this case, in fact, the intention is to meet a grave spiritual need for the eternal salvation of an individual believer, not to bring about an intercommunion which remains impossible until the visible bonds of ecclesial communion are fully re-established.

This was the approach taken by the Second Vatican Council when it gave guidelines for responding to Eastern Christians separated in good faith from the Catholic Church, who spontaneously ask to receive the Eucharist from a Catholic minister and are properly disposed.95 This approach was then ratified by both Codes, which also consider – with necessary modifications – the case of other non-Eastern Christians who are not in full communion with the Catholic Church.96

46. In my Encyclical Ut Unum Sint I expressed my own appreciation of these norms, which make it possible to provide for the salvation of souls with proper discernment: “It is a source of joy to note that Catholic ministers are able, in certain particular cases, to administer the sacraments of the Eucharist, Penance and Anointing of the Sick to Christians who are not in full communion with the Catholic Church but who greatly desire to receive these sacraments, freely request them and manifest the faith which the Catholic Church professes with regard to these sacraments. Conversely, in specific cases and in particular circumstances, Catholics too can request these same sacraments from ministers of Churches in which these sacraments are valid”.97

These conditions, from which no dispensation can be given, must be carefully respected, even though they deal with specific individual cases, because the denial of one or more truths of the faith regarding these sacraments and, among these, the truth regarding the need of the ministerial priesthood for their validity, renders the person asking improperly disposed to legitimately receiving them. And the opposite is also true: Catholics may not receive communion in those communities which lack a valid sacrament of Orders.98

The faithful observance of the body of norms established in this area 99 is a manifestation and, at the same time, a guarantee of our love for Jesus Christ in the Blessed Sacrament, for our brothers and sisters of different Christian confessions – who have a right to our witness to the truth – and for the cause itself of the promotion of unity.