"Wherefore I say unto you: all manner of sin and blasphemy shall be forgiven unto men: but the blasphemy against the Holy Ghost shall not be forgiven unto men." (Matthew 12:31)
"But he that shall blaspheme against the Holy Ghost hath never forgiveness, but is in danger of eternal damnation." (Mark 3:29)
What is the sin that shall not be forgiven? Nobody knows, though footnotes to the bible, which conflict each other, try to make people feel like this passage is not something believers should worry about. It is all quite obscure; and that's an area where you want a lot of obscurity -- on the subject of the secret crime against god that cannot be forgiven. One of my favorite bits in the bible.