1 Corinthians 11:29
For he that eateth and drinketh unworthily, eateth and drinketh damnation to himself, not discerning the Lord’s body. For he that eateth and drinketh unworthily, eateth and drinketh damnation to himself, not discerning the Lord’s body.
At Corinth the warning was given because the sacred ordinance had been corrupted into selfish feasting with disregard for fellow believers. To discern the Lord's body is to recognize what the bread and the cup recall, the broken bread His broken body and the juice of the vine His spilled blood. It is to recognize Christ who gave Himself for the life of the world, and to keep His love and His sacrifice fresh in memory in the communion service. The Jews illustrate failure to discern, for they made the Passover lamb and its service important while missing Him to whom it pointed.
Whoever eats and drinks unworthily without that recognition eats and drinks judgment and condemnation and damnation to himself. Inwardly false participants, like a Judas appearing for a time to be with believers, partake to their own condemnation. Their hypocrisy is not hidden, for God quickly makes them manifest and purges them out and removes them from among His people. The warning places the responsibility on each person to examine himself, and it does not authorize others to judge who may come, except where open sin is involved.
Worthy reception requires a heart ready for fellowship with Christ, which pride and conflict and rivalry prevent. Christ therefore appointed the ordinance of foot washing as preparation for communion, so that His lesson of humility and service would be remembered first. Following His pattern of taking His disciples away alone separate from the wicked, first washing their feet and then giving them the emblems, the washing of feet and the communion are not to be held among unbelievers but apart by themselves, and shepherds must be careful to move understandingly in this. Participation in foot washing is nevertheless not required before receiving communion, so a believer who had not taken part in it is still to be welcomed to the Lord's Supper.