Purgatory
1 Corinthians 3:13-15
Every man's work shall be manifest... the fire shall try every man's work, of what sort it is... If any man's work burn, he shall suffer loss: but he himself shall be saved, yet so as by fire.
Context of the passage
Paul describes how the quality of each believer's works, built upon the foundation of Christ, will be tested and revealed on the Day of Judgment.
Catholic reading
Paul describes a person who is ultimately saved yet still suffers a purifying loss 'as by fire' for imperfections in his works, a scriptural foundation for the doctrine of Purgatory as a final cleansing before entering the fullness of glory.
Common objection
This 'fire' refers only to a momentary test at the final judgment revealing the worth of one's works, not an intermediate state of purification after death.
Response
Even on a purely eschatological reading, the passage still establishes the key Catholic principle at stake: that a truly saved person can nonetheless undergo a real, painful purification for imperfect works, which is precisely the theological reality Purgatory names, whether experienced at particular or general judgment.