Are atheists sinless?

This is an odd question, but I know that from a christian perspective, unintentional sins aren't really considered sins. If someone doesn't know that their actions are sinful, then they aren't sinning. So this brings up an interesting question about atheists. Atheists don't believe in sin because no action would be a transgression against a god who isn't even there, right? So if they don't believe that their actions are sinful, then they would never sin. I mean, I know a sin's a sin no matter who commits it, but it's not like they can be held accountable for those sins since they don't believe they're sins. Now, I'm sure that some of you will say that it's a sin to reject God, but why exactly is that a sin? People don't choose what they believe, they just believe what makes sense to them. And it's not like it's impossible for someone to not believe in a religion, since there isn't any definitive solid tangible proof for any religion anyway. So if what someone believes makes sense, then couldn't God respect that they are making what they believe to be the most rational decision, and wouldn't they then be exempt from the sin of rejecting him? Any insight is greatly appreciated.