17. Through our communion in his body and blood, Christ also grants us his Spirit. Saint Ephrem writes: He called the bread his living body and he filled it with himself and his Spirit…
He who eats it with faith, eats Fire and Sprit… Take and eats this, all of you, and eat with it the Holy Spirit. For it is truly my body and whoever eats it will have eternal life. The Church implores this divine Gift, the source of every other gift, in the Eucharistic epiclesis. In the Divine Liturgy of Saint John Chrysostom, for example, we find the prayer: We beseech, implore and beg you: send your Holy Spirit upon us all and upon these gifts… that those who partake of them may be purified in soul, receive the forgiveness of their sins, and share in the Holy Spirit. And in the Roman Missal the celebrant prays: grant that we who are nourished by his body and blood may be filled with his Holy Spirit, and become one body, one spirit in Christ. Thus by the gift of his body and blood Christ increases within us the gift of his Spirit, already poured out in baptism and bestowed as a seal in the sacrament of Confirmation.

20 ….. Many problems darken the horizon of our time. We need but think of the urgent need to work for peace, to base relationships between peoples on solid premises of justice and solidarity, and to defend human life from conception to its natural end. And what should we say of the thousand inconsistencies of a globalized world where the weakest, the most powerless and the poorest appear to have so little hope! It is in this world that Christian hope must shine forth! For this reason too, the Lord wished to remain with us in the Eucharist, making his presence in meal and sacrifice the promise of a humanity renewed by his love. Significantly, in their account of the Last Supper, the Synoptics recount the institution of the Eucharist, while the Gospel of John relates, as a way of bringing out its profound meaning the account of the washing of the feet, in which Jesus appears as the teacher of communion and of service (cf. In 13:1-20). The Apostle Paul, from his part, says that it is unworthy of a Christian community to partake of the Lord's Supper amid division and indifference towards the poor (cf. 1 Cor 11:17-22, 27-34).
Proclaiming the death of the Lord until he comes (1 Cor 11:26) entails that all who take part in the Eucharist be committed to changing their lives and making them in a certain way completetly Eucharistic. It is this fruit of a transfigured existence and a commitment to transforming the world in a accordance with the Gospel which splendidly illustrates the eschatological tension inherent in the celebration of the Eucharist and in the Christian life as a whole: Come, Lord Jesus! (Rev 22:20).

43 ….. 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, inasmuch as it is the supreme sacrament 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. 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. (from the Encyclical Letter of His Holiness Pope John Paul II, Ecclesia de Eucharistia)