Purgatory
2 Maccabees 12:44-46
For if he had not hoped that they that were slain should rise again, it would have seemed superfluous and vain to pray for the dead... It is therefore a holy and wholesome thought to pray for the dead, that they may be loosed from sins.
Context of the passage
After a battle, Judas Maccabeus discovers slain soldiers had secretly worn pagan amulets and orders a sin offering and prayers for them in Jerusalem.
Catholic reading
This passage explicitly commends praying and offering sacrifice for the dead 'that they may be loosed from sins,' presupposing an intermediate state where the dead can still be aided and purified, the clearest Old Testament foundation for the doctrine of Purgatory.
Common objection
Maccabees is not part of the Protestant biblical canon, so this text carries no binding scriptural authority for establishing doctrine.
Response
2 Maccabees was part of the Septuagint used by the apostles and the early Church, was read as Scripture by figures like Augustine, and was affirmed as canonical at councils such as Rome (382) and Carthage (397), long before the 16th-century removal of the deuterocanon by Protestant reformers.