I deny the Holy Spirit
The one sin that the Christian God apparently cannot ever forgive is blasphemy against the Holy Spirit.
Verily I say unto you, All sins shall be forgiven unto the sons of men, and blasphemies wherewith soever they shall blaspheme: But he that shall blaspheme against the Holy Ghost hath never forgiveness, but is in danger of eternal damnation. -- John 3:28-29
The website Blasphemy Challenge is asking people to create videos of themselves committing the unpardonable sin and uploading it to YouTube. They get a free DVD of 'The God Who Wasn't There' (only if they live in the US). I've already downloaded the movie (although I haven't watched it yet), but I'd still take the challenge. Sadly, I don't have any means to video myself.
So, for the record, I would like to state the following:
I completely and utterly deny the existence of God, Jesus & the Holy Spirit.