The Holy Eucharist is a sacrament and a sacrifice. In the Holy Eucharist, under the appearances of bread and wine, the Lord Christ is contained, offered, and received.
(a) The whole Christ is really, truly, and substantially present in the Holy Eucharist. We use the words "really, truly, and substantially" to describe Christ's presence in the Holy Eucharist in order to distinguish Our Lord's teaching from that of mere men who falsely teach that the Holy Eucharist is only a sign or figure of Christ, or that He is present only by His power.
(b) All Christians, with but few minor exceptions, held the true doctrine of the Real Presence from the time of Christ until the Protestant Revolution in the sixteenth century.
(c) The word "Eucharist" means "Thanksgiving."
When did Christ institute the Holy Eucharist?
Christ instituted the Holy Eucharist at the Last Supper, the night before He died.
About a year before the Last Supper Our Lord promised to give us the Holy Eucharist. This promise is related in the sixth chapter of the Gospel according to Saint John. The fulfillment of this promise took place at the Last Supper.
How did Christ institute the Holy Eucharist?
Christ instituted the Holy Eucharist in this way: He took bread, blessed and broke it, and giving it to His apostles, said: "Take and eat; this is My body;" then He took a cup of wine, blessed it, and giving it to them, said: "All of you drink of this; for this is My blood of the new covenant which is being shed for many unto the forgiveness of sins;" finally, He gave His apostles the commission: "Do this in remembrance of Me."
What happened when Our Lord said: "This is My body . . . this is My blood"?
When Our Lord said, "This is My body," the entire substance of the bread was changed into His body; and when He said, "This is My blood," the entire substance of the wine was changed into His blood.
Christ could not have used clearer, more explicit words than "This is My body." He did not say, "This is a sign of My body," or "This represents My body," but, "This is My body." Catholics take Christ at His word because He is the omnipotent God. On His word they know that the Holy Eucharist is the body and blood of Christ.