There are no sins to the atheist?
Stripey, you've asked a very good question, and I am happy to give you my opinion...
The very concept of sin comes from religious guidebooks. Religions offer to solve a problem of their own making! (Sounds like George Bush and 9/11, doesn't it?)
I would not be thankful to a person who cut me with a knife to sell me a bandage. I would not be respectful nor return to a doctor who makes me sick in order to stay in business. I'm not going to vote for a president who has lied to win approval for a war that he wanted all along.
Instead of "I am guilty but forgiven," I prefer "I was innocent all along." Sin is a viscious concept. It keeps people subservient. Even Jesus was supposed to have said, "They who are not sick need no physician."
Also, even though Christians are supposedly forgiven for all transgressions, they are never free
from sin. Temptation lurks in every idle moment. How happy can one be if one thinks that every action and thought is being monitored by a ghostly judge?
Of course, Christians will say that paying daily homage to that ghostly judge brings them joy. Well, sure...it's like the joy of a slave who has gotten through the day without being beaten. An abused child has gotten past the angry parent without receiving a hard smack. A worker has gotten to the end of his shift without his boss giving him a lecture in front of his colleagues. (See Dan Barker's book Losing Faith in Faith
for more on this topic.)
But is that real joy? To me, the real joy would come from eliminating the oppression altogether.