What is the Mass?

The Mass is the sacrifice of the New Law in which Christ, through the ministry of the priest, offers Himself to God in an un-bloody manner under the appearances of bread and wine.

Who said the first Mass?

Our Divine Savior said the first Mass, at the Last Supper, the night before he died. At the Last Supper, Jesus Christ offered Himself up as a sacrifice to the Eternal Father, under the appearances of bread and wine. The following day, Jesus Christ consummated that Sacrifice by freely submitting Himself to His Passion and death by crucifixion at the hands of the Jews.

The Mass is a real sacrifice, for in it a Victim is offered up for the purpose of reconciling man with God. Our Lord caused His passion and death to enter into the institution of the Mass, thereby joining them as one.

Why is the Mass the same sacrifice as the Sacrifice of the Cross?

The Mass is the same sacrifice as the sacrifice of the cross, because in the Mass the victim is the same, and the principal Priest is the same, Jesus Christ. The Mass is the very same sacrifice which was offered up at the Last Supper, and consummated on Calvary; it is the living renewal of the sacrifice of the cross. The Mass is no mere remembrance or memorial of Calvary; it actually renews, in the separate consecration of the bread and wine, the death of the Lord, the separation of His Body and Blood. The principal priest in every Mass is Jesus Christ, who offers to His heavenly Father, through the ministry of His ordained priest, His body and blood which were sacrificed on the cross.

What is the Holy Eucharist?

The Holy Eucharist is a sacrament and a sacrifice in which Our Savior Jesus Christ, body and blood, soul and divinity, is contained, offered, and received under the appearances of bread and wine. Christ instituted the Holy Eucharist at the last supper the night before He died. 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. After the substance of the bread and wine had been changed, only the appearances of bread and wine remained. (By the appearances of bread and wine we mean their color, taste, weight, shape, and whatever else appears to the senses.)

Why do we believe that Christ changed bread and wine into His own Body and Blood?

We believe that Christ changed bread and wine into His own Body and Blood, because:

1. His words clearly say so. At the Last Supper He said "This is My Body," not "This is a symbol of My Body," or "This represents My Body."
2. Previously, on the day after the first multiplication of the loaves and fishes, Our Lord had promised to give His Flesh to eat and His Blood to drink. On this occasion, it is clear that the Jews took Our Lord's words literally. Many of the disciples left Jesus and "walked no more with Him," because they could not believe such a thing as He promised. But Jesus, although very sad at their leaving, did not take back his words or explain them differently. "Amen, amen, I say to you, Unless you eat the flesh of the Son of Man, and drink his blood, you shall not have life in you. He who eats my flesh and drinks my blood has life everlasting and I will raise him up on the last day. For my flesh is food indeed, and my blood is drink indeed." (John 6:54-56).
3. The Apostles understood that Christ meant His words at the Last Supper to be literal. St. Paul writes: "The cup of blessing that we bless, is it not the sharing of the blood of Christ? and the bread that we break, is it not the partaking of the body of the Lord?... Therefore Whoever eats this bread or drinks the cup of the Lord unworthily, will be guilty of the body and the blood of the Lord. But let a man prove himself, and so let him eat of that bread and drink of the cup' for he who eats and drinks unworthily, without distinguishing the body, eats and drinks judgement to himself." (1 Cor. 10:16; 11:27-29).
4. It has been the continuous belief of Christians from the beginning of Christianity. St. Augustine said, 'Our lord held Himself in His own hands, when He gave His Body to the disciples." It was only in the 16th century  that Protestants, breaking away from the True Church, denied it and introduced a different doctrine.