Question:
Answer:
Your premise is wrong. Venial sins can be forgiven in Purgatory (CCC 1030-32).
Jesus teaches us that some sins cannot be forgiven in the next life (unrepented mortal sins, which will separate a person from God for all eternally), but some can be forgiven (venial sins):
And whoever says a word against the Son of man will be forgiven; but whoever speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come (Matthew 12:32, emphasis added).
This statement of our Lord implies there are at least some sins that can be forgiven after death, whether at one’s particular judgment or in Purgatory. Those would be venial sins.
For more on Purgatory, please see our tract on that subject. Please see also Tim Staple’s article on “Mortal and Venial Sin?”