Our lesson from the Heidelberg Catechism today explained the differences between our celebration of the Lord’s Supper and the Roman Catholic Mass.
One of these differences is that the Mass is considered a genuine sacrifice offering of Christ upon the altar by a priest. That Christ is in some way, re-offered and re-sacrificed to God, and that ongoing participation in the Mass is then necessary for the forgiveness of sins. This error is so great that the Heidelberg Catechism – which otherwise tends to have a warm and congenial tone – calls it an idolatry that must be condemned.
The reason for this is that the teaching of the Mass flies in the face of the teaching of Scripture, especially the Book of Hebrews. For in Hebrews, we read that unlike the priests of old who had to stand daily and repeatedly offer sacrifices, Christ has offered for all time a single sacrifice for sins. And having done so, He is no longer standing, but is seated at the right hand of God.
Christ’s one perfect sacrifice is all His saints need.
Under the Old Covenant, we needed many priests and many sacrifices, but in this new and better Covenant, we have a great High Priest who by His one sacrifice of Himself intercedes for us forever.
So at this Table, I am not in any way sacrificing Christ. To do so would be to deny His great work. Rather, I am putting forth before you a visible token and pledge of His perfect offering on your behalf.
So come, and welcome, to Jesus Christ.
 Q. 80. What difference is there between the Lord’s Supper and the papal Mass?
Answer: The Lord’s Supper testifies to us that we have complete forgiveness of all our sins through one sacrifice of Jesus Christ which he himself has accomplished on the cross once for all; (and that through the Holy Spirit we are incorporated into Christ, who is now in heaven with his true body at the right hand of the Father and is there to be worshipped). But the Mass teaches that the living and the dead do not have forgiveness of sins through the sufferings of Christ unless Christ is again offered for them daily by the priest (and that Christ is bodily under the form of bread and wine and is therefore to be worshipped in them). Therefore the Mass is fundamentally a complete denial of the once for all sacrifice and passion of Jesus Christ (and as such an idolatry to be condemned).