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.
The whole Christ is really, truly, and substantially present in the Holy Eucharist.
The word “Eucharist” means "Thanksgiving and Christ instituted the Holy Eucharist at the Last Supper, the night before He died.
When Our Lord instituted the Holy Eucharist the apostles were present, it is not fabrication.
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.”
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.
The change of the entire substance of the bread and wine into the body and blood of Christ is called Transubstantiation.
Jesus Christ is whole and entire both under the appearances of bread and under the appearances of wine.
Our Lord was able to change bread and wine into His body and blood by His almighty power.God, who created all things from nothing, who fed the five thousand with five loaves, who changed water into wine instantaneously, who raised the dead to life, can change bread and wine into the body and blood of Christ. Although the Holy Eucharist is a great mystery, and consequently beyond human understanding, the principles of sound reason can show that this great gift is not impossible by the power of God.