1 Cor. 11:27-34 (ESV)
Whoever, therefore, eats the bread or drinks the cup of the Lord in an unworthy manner will be guilty of profaning the body and blood of the Lord.  Let a person examine himself, then, and so eat of the bread and drink of the cup.  For anyone who eats and drinks without discerning the body eats and drinks judgment on himself.  That is why many of you are weak and ill, and some have died.  But if we judged ourselves truly, we would not be judged.  But when we are judged by the Lord, we are disciplined so that we may not be condemned along with the world.
 So then, my brothers, when you come together to eat, wait for one another—  if anyone is hungry, let him eat at home—so that when you come together it will not be for judgment. About the other things I will give directions when I come.
The Lord’s Supper is not something to be taken lightly. It is the gospel itself, the most sacred of sacred, it is a participation in the most holy sacrifice ever made, the sacrifice of Christ’s holy Body and Blood on the Cross. The Lord’s Supper is Holy. To take it in an unworthy manner is to profane the sacred. To profane means to treat something as profane, meaning common, or not holy. Profane things are not bad in and of themselves they are merely not holy. The meals we eat at home are profane, they are common. For this reason we have no qualms with sharing our meals with whomever we please. But it is a sin to treat the holy things of God as if they were not holy. That is to not give them the proper respect they deserve. One can do this in many ways. We can approach the Lord’s Supper unrepentant of our sin. This is something we have to leave for the most part to the individual, if their sin is not open and manifest. But if they say they are repentant of it, and do not want to return to it, we have no choice but to believe them. Only God can search a person’s heart. We can, as the Corinthians did, treat the Lord’s Supper as any other meal we might have during the week, and serve it to anyone regardless of their professed beliefs, either concerning God or concerning the nature of the supper. False belief is also a sin. Or we can fail to discern the body; the word here means to distinguish it. This means that those who do not believe it is anything more than bread and or wine, are not discerning that it is the body of Christ. In this case, he does not receive the blessing Christ attaches to His body and blood, but eats and drinks judgment on himself. Those who do not believe it is the body and blood, are actually calling Christ a liar. They may be doing so unwittingly, but they are doing it nonetheless. We have a duty to correct such unbelief.