What does it mean to sin "against the body and blood of the Lord" (verse 27). It means to commit an act that is disrespectful of this holy meal, in this case, by eating of it in a careless fashion, without a thought of slighting others in the congregation. The Lord's Table, as it is called in 10:21, is holy and demands our reverence. If we sin carelessly and then expect to partake of the elements depicting Christ's suffering and death for our sins, we are hypocrites. In the Old Testament, the priests had become casual towards holy things and were rebuked for it:
"You place defiled food on my altar.
But you ask, 'How have we defiled you?'
By saying that the LORD's table is contemptible." (Malachi 1:7)
As terrible as it is to sin "against the body and blood of the Lord," we must realize that it is no more or less sinful than other sins against fellow Christians. And all are sins against Christ himself (8:12; Matthew 25:31-46).