Eucharist
1 Corinthians 11:27-29
Whosoever shall eat this bread, or drink the chalice of the Lord unworthily, shall be guilty of the body and of the blood of the Lord... he that eateth and drinketh unworthily, eateth and drinketh judgment to himself, not discerning the body of the Lord.
Context of the passage
Paul corrects abuses at Corinthian communal meals where the Eucharist was received carelessly amid factions.
Catholic reading
One cannot be 'guilty of the body and blood of the Lord' by mishandling mere bread; the severity implies the Eucharist truly is Christ's body, grounding the Church's practice of examining conscience before Communion.
Common objection
The 'body' here might just mean the Church community, sinned against by disunity.
Response
Verse 29 explicitly says 'discerning the body of the Lord,' referring back to the bread just mentioned, not an abstract community; the whole passage concerns the sacramental elements themselves.