"At the Last Supper, on the night when He was betrayed, our Saviour instituted the Eucharistic sacrifice of His Body and Blood. This He did in order to perpetuate the sacrifice of the Cross throughout the centuries until He should come again, and so to entrust to His beloved spouse, the Church, a memorial of His death and resurrection: a sacrament of love, 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.”
The Holy Mass is the sacrifice in which the Lord Jesus Christ, through the ministry of the priest, offers Himself to God the Father in an un-bloody manner, under the appearances of bread and wine, for the salvation of our souls. The Second Vatican Ecumencial Council Constitution on the Sacred Liturgy Sacrosanctum Concilium, n. 47
"As early as the second century we have the witness of St. Justin Martyr for the basic lines of the order of the Eucharistic celebration. They have stayed the same until our own day for all the great liturgical families. St. Justin wrote to the pagan emperor Antoninus Pius (138-161) around the year 155, explaining what Christians did:
Every Mass makes present the Death and Resurrection of Our Lord, transcending space and time. Every Mass brings new graces into the world. During every Mass, Christ Himself becomes present on the Altar, as through the words of a priest and the action of the Holy Spirit, our gifts of bread and wine are really, truly, and substantially changed into the Body, Blood, Soul, and Divinity of Jesus Christ Himself. This is the most incredible reality in the whole world.
The Catechism of the Catholic Church continues:
"The mode of Christ's presence under the Eucharistic species is unique. It raises the Eucharist above all the sacraments as "the perfection of the spiritual life and the end to which all the sacraments tend." In the most blessed sacrament of the Eucharist 'the body and blood, together with the soul and divinity, of our Lord Jesus Christ and, therefore, the whole Christ is truly, really, and substantially contained.' 'This presence is called 'real' - by which is not intended to exclude the other types of presence as if they could not be 'real' too, but because it is presence in the fullest sense: that is to say, it is a substantial presence by which Christ, God and man, makes himself wholly and entirely present.'
It is by the conversion of the bread and wine into Christ's body and blood that Christ becomes present in this sacrament. The Church Fathers strongly affirmed the faith of the Church in the efficacy of the Word of Christ and of the action of the Holy Spirit to bring about this conversion.
Thus St. John Chrysostom declares: 'It is not man that causes the things offered to become the Body and Blood of Christ, but he who was crucified for us, Christ himself. The priest, in the role of Christ, pronounces these words, but their power and grace are God's. 'This is my body', he says. This word transforms the things offered.
And St. Ambrose says about this conversion: Be convinced that this is not what nature has formed, but what the blessing has consecrated. The power of the blessing prevails over that of nature, because by the blessing nature itself is changed. . . . Could not Christ's word, which can make from nothing what did not exist, change existing things into what they were not before? It is no less a feat to give things their original nature than to change their nature.
The Council of Trent summarizes the Catholic faith by declaring: 'Because Christ our Redeemer said that it was truly his body that he was offering under the species of bread, it has always been the conviction of the Church of God, and this holy Council now declares again, that by the consecration of the bread and wine there takes place a change of the whole substance of the bread into the substance of the body of Christ our Lord and of the whole substance of the wine into the substance of his blood. This change the holy Catholic Church has fittingly and properly called transubstantiation.'" (CCC 1374-1376)