Mark 3:29 But he that shall blaspheme against the Holy Ghost hath never forgiveness, but is in danger of eternal damnation:
Basically atheism is one thing you can never be forgiven for and will make you live your eternal afterlife in a lake of fire. You can kill, rape, and torture as much as you want. As long as you accept the Jesus myth and genuinely ask for his forgiveness, you can still get a ticket to paradise.
The guys who thought to add this clause were very clever. They realized no one could ever live by this crap and everyone would sin. By having God/Jesus forgive you for your sins, you can convince the faithful the ticket to paradise is always available, they just always need to try harder. Conversely, if you deny the existence of God, you'll never be forgiven and get a one-way ticket to hell. (My ticket's punched.)
It would have been interesting to be a fly on the wall when they came up with this one.
"Mark, so how can we make it suck if people don't believe?"...
"I know, we'll just make it the one unforgivable sin!"
"Oh good one, write that down now."