The Heidelberg Catechism: Lord's Day 30
80. Question: What difference is there between the Lord's supper and the papal mass?
Answer: The Lord's supper testifies to us, first, that we have complete forgiveness of all our sins through the one sacrifice of Jesus Christ, which He Himself accomplished on the cross once for all; and, second, that through the Holy Spirit we are grafted into Christ, who with His true body is now in heaven at the right hand of the Father, and this is where He wants to be worshipped. But the mass teaches, first, that the living and the dead do not have forgiveness of sins through the suffering of Christ unless He is still offered for them daily by the priests; and, second, that Christ is bodily present in the form of bread and wine, and there is to be worshipped. Therefore the mass is basically nothing but a denial of the one sacrifice and suffering of Jesus Christ, and an accursed idolatry.
 Matt. 26:28; John 19:30; Heb. 7:27; 9:12, 25, 26; 10:10-18.  I Cor. 6:17; 10:16, 17.  Joh. 20:17; Acts 7:55, 56; Heb. 1:3; 8:1.  John 4:21-24; Phil. 3:20; Col. 3:1; I Thess. 1:10.
81. Question: Who are to come to the table of the Lord?
Answer: Those who are truly displeased with themselves because of their sins and yet trust that these are forgiven them and that their remaining weakness is covered by the suffering and death of Christ, and who also desire more and more to strengthen their faith and amend their life. But hypocrites and those who do not repent eat and drink judgment upon themselves.
 I Cor. 10:19-22; 11:26-32.
82. Question: Are those also to be admitted to the Lord's supper who by their confession and life show that they are unbelieving and ungodly?
Answer: No, for then the covenant of God would be profaned and His wrath kindled against the whole congregation. Therefore, according to the command of Christ and His apostles, the Christian church is
duty-bound to exclude such persons by the keys of the kingdom of heaven, until they amend their lives.
 Ps. 50:16; Is. 1:11-17; I Cor. 11:17-34.