The Holy Eucharist is the Sacrament which contains the body and blood, soul and divinity, of our Lord Jesus Christ under the appearances of bread and wine. Christ instituted the Holy Eucharist at the Last Supper, the night before He died. When our Lord instituted the Holy Eucharist the twelve Apostles were present. Our Lord instituted the Holy Eucharist by taking bread, blessing, breaking, and giving to His Apostles, saying: Take ye and eat. This is My body; and then by taking the cup of wine, blessing and giving it, saying to them: Drink ye all of this. This is My blood which shall be shed for the remission of Sins. Do this for a commemoration of Me. When our Lord said, This is My body, the substance of the bread was changed into the substance of His body; when He said, This is My blood, the substance of the wine was changed into the substance of His blood. Jesus Christ is whole and entire both under the form of bread and under the form of wine.
After the substance of the bread and wine had been changed into the substance of the body and blood of our Lord there remained only the appearances of bread and wine, which means that visibly, and to all our senses, the bread and wine looked and tasted the same as before it was consecrated. The substance of the bread and wine was changed into the substance of the body and blood of Christ by His almighty power. This change of bread and wine into the body and blood of Christ continues to be made in the Church by Jesus Christ through the ministry of His priests. Christ gave His priests the power to change bread and wine into His body and blood when He said to the Apostles, Do this in commemoration of Me. The priests exercise this power of changing bread and wine into the body and blood of Christ through the words of consecration in the Mass, which are the words of Christ: This is My body; this is My blood.