The Eucharist is the very sacrifice of the Body and Blood of the Lord Jesus which he instituted to perpetuate the sacrifice of the cross throughout the ages until his return in glory. Thus he entrusted to his Church this memorial of his death and Resurrection. It is a sign of unity, a bond of charity, a paschal banquet, in which Christ is consumed, the mind is filled with grace, and a pledge of future glory is given to us.
Jesus instituted the Eucharist on Holy Thursday “the night on which he was betrayed”
(1 Corinthians 11:23), as he celebrated the Last Supper with his apostles.
After he had gathered with his apostles in the Cenacle, Jesus took bread in his hands. He broke it and gave it to them saying, “Take this and eat it, all of you; this is my Body which will be given up for you”. Then, he took the cup of wine in his hands and said, “Take this and drink of this, all of you. This is the cup of my Blood, the Blood of the new and everlasting covenant. It will be shed for you and for all so that sins may be forgive. Do this in memory of me”.
It is the source and summit of all Christian life. In the Eucharist, the sanctifying action of God in our regard and our worship of him reach their high point. It contains the whole spiritual good of the Church, Christ himself, our Pasch. Communion with divine life and the unity of the People of God are both expressed and effected by the Eucharist. Through the eucharistic celebration we are united already with the liturgy of heaven and we have a foretaste of eternal life.
The unfathomable richness of this sacrament is expressed in different names which evoke its various aspects. The most common names are: the Eucharist, Holy Mass, the Lord’s Supper, the Breaking of the Bread, the Eucharistic Celebration, the Memorial of the passion, death and Resurrection of the Lord, the Holy Sacrifice, the Holy and Divine Liturgy, the Sacred Mysteries, the Most Holy Sacrament of the Altar, and Holy Communion.
The Eucharist was foreshadowed in the Old Covenant above all in the annual Passover meal celebrated every year by the Jews with unleavened bread to commemorate their hasty, liberating departure from Egypt. Jesus foretold it in his teaching and he instituted it when he celebrated the Last Supper with his apostles in a Passover meal. The Church, faithful to the command of her Lord, “Do this in memory of me” (1 Corinthians 11:24), has always celebrated the Eucharist, especially on Sunday, the day of the Resurrection of Jesus.
The Eucharist unfolds in two great parts which together form one, single act of worship. The Liturgy of the Word involves proclaiming and listening to the Word of God. The Liturgy of the Eucharist includes the presentation of the bread and wine, the prayer or the anaphora containing the words of consecration, and communion.
The Eucharist is a memorial in the sense that it makes present and actual the sacrifice which Christ offered to the Father on the cross, once and for all on behalf of mankind. The sacrificial character of the Holy Eucharist is manifested in the very words of institution, “This is my Body which is given for you” and “This cup is the New Covenant in my Blood that will be shed for you” (Luke 22:19-20). The sacrifice of the cross and the sacrifice of the Eucharist are one and the same sacrifice. The priest and the victim are the same; only the manner of offering is different: in a bloody manner on the cross, in an unbloody manner in the Eucharist.
In the Eucharist the sacrifice of Christ becomes also the sacrifice of the members of his Body. The lives of the faithful, their praise, their suffering, their prayers, their work, are united to those of Christ. In as much as it is a sacrifice, the Eucharist is likewise offered for all the faithful, living and dead, in reparation for the sins of all and to obtain spiritual and temporal benefits from God. The Church in heaven is also united to the offering of Christ.
Jesus Christ is present in the Eucharist in a unique and incomparable way. He is present in a true, real and substantial way, with his Body and his Blood, with his Soul and his Divinity. In the Eucharist, therefore, there is present in a sacramental way, that is, under the Eucharistic species of bread and wine, Christ whole and entire, God and Man.
Transubstantiation means the change of the whole substance of bread into the substance of the Body of Christ and of the whole substance of wine into the substance of his Blood. This change is brought about in the eucharistic prayer through the efficacy of the word of Christ and by the action of the Holy Spirit. However, the outward characteristics of bread and wine, that is the “eucharistic species”, remain unaltered.
Source: Compendium of the Catechism of the Catholic Church