Heidelberg Catechism lesson 30

Q. 80. What difference is there between the Lord’s Supper and the popish mass?
The Lord’s Supper testifies to us, that we have a full pardon of all sin by the only sacrifice of Jesus Christ, which He Himself has once accomplished on the cross;[1] and, that we by the Holy Ghost are ingrafted into Christ,[2] who, according to His human nature is now not on earth, but in heaven, at the right hand of God His Father,[3] and will there be worshipped by us:[4]—but the mass teaches, that the living and dead have not the pardon of sins through the sufferings of Christ, unless Christ is also daily offered for them by the priests; and further, that Christ is bodily under the form of bread and wine, and therefore is to be worshipped in them; so that the mass, at bottom, is nothing else than a denial of the one sacrifice and sufferings of Jesus Christ, and an accursed idolatry.[5]
[1] Hebrews 7:27 and 9:12, 26; Matthew 26:28; Luke 22:19–20; 2 Corinthians 5:21;  [2] 1 Corinthians 6:17 and 12:13;  [3] Hebrews 1:3 and 8:1, &c.;   [4] John 4:21–23; Colossians 3:1; Philippians 3:20; Luke 24:52–53; Acts 7:55;  [5] Isaiah 1:11, 14; Matthew 15:9; Colossians 2:22–23; Jeremiah 2:13.
Q. 81. For whom is the Lord’s Supper instituted?
For those who are truly sorrowful for their sins, and yet trust that these are forgiven them for the sake of Christ;[1] and that their remaining infirmities are covered by His passion and death;[2] and who also earnestly desire to have their faith more and more strengthened, and their lives more holy; [3] but hypocrites, and such as turn not to God with sincere hearts, eat and drink judgment to themselves.[4]
[1] Matthew 5:3, 6; Luke 7:37–38 and 15:18–19;  [2] Psalm 103:3;  [3] Psalm 116:12–14; 1 Peter 2:11–12;  [4] 1 Corinthians 10:20, &c., and 11:28, &c.; Titus 1:16; Psalm 50:15–16.
Q. 82. Are they also to be admitted to this supper, who, by confession and life, declare themselves unbelieving and ungodly?
No; for by this, the covenant of God would be profaned, and His wrath kindled against the whole congregation;[1] therefore it is the duty of the Christian church, according to the appointment of Christ and His Apostles, to exclude such persons, by the keys of the kingdom of heaven, till they show amendment of life.[2]
[1] 1 Corinthians 10:21 and 11:30–31; Isaiah 1:11, 13; Jeremiah 7:21; Psalm 50:16, 22;   [2] Matthew 18:17–18.
The popish mass is one of the most obnoxious inventions of Romanism. While we can allow that transubstantiation, though superstitious and repugnant to reason, can be obtusely argued from the Scripture, the mass (which slipped into the church by way of transubstantiation) is pure pagan idolatry. Wilhelmus à Brakel describes it this way:

This they call the mass, in which the celebrant (whom they call the priest) stands before a table (which they call an altar) decorated with silver, gold, and other physical ostentations, and with images, crosses, and burning candles (even during the middle of the day). He furthermore performs many ridiculous and ludicrous ceremonies such as the removal of a book from one location to the other, the making of knee-bends, the repeated overturning of stones, the ringing of bells, and a muttering behind his garment which he has lifted up from behind him. Last of all, he makes a Christ, that is, a God, out of his wafer, which he then lifts above his head and shows to those who are present for the purpose of worship. This they do while bending their knees and smiting upon their breasts with great reverence. After the bread-god has been worshipped, he breaks him in pieces with a feigned trembling of his limbs—as if he were terrified. He then consumes him, upon which he empties the cup with one draught, having made its wine into the blood of his God. This then is a sacrifice for the forgiveness of sins, be it for the living or for the souls in purgatory who are strengthened by it as a hungry person is strengthened who dreams that he eats. Having concluded this, he declares, “Ita missa est,” that is, this is in return for your money (Wilhelmus à Brakel, The Christian’s Reasonable Service, 2.535–536).

Need we even add that “Christ was once offered to bear the sins of many” (Heb 9:28; cf. 7:27).

Q/A. 81–82 have to do with the doctrine and practice of restricting the Table. Due to rampant individualism, most churches today practise “open communion,” and some even argue that the church has no right to exclude anyone from the Lord’s Table. They say that Paul only requires personal self-examination. But they who argue this way fail to realise that the Lord’s Supper is a corporate rather than individual exercise, for the Apostle Paul says: “The cup of blessing which we bless, is it not the communion of the blood of Christ? The bread which we break, is it not the communion of the body of Christ? For we being many are one bread, and one body: for we are all partakers of that one bread” (1 Cor 10:16–17). And, therefore, when members of the congregation partake of the Lord’s Supper unworthily, we can expect the chastisement of the Lord (1 Cor 11:30–31) to fall upon the whole congregation. This, plus the fact that the standard of morality even amongst Christians today is widely varying, demands that the elders of the church exclude from the Lord’s Supper, such as are under church discipline (Mt 18:17–18), or are visitors who are unknown to the congregation.