"To them who know what is right, and refuse it still, it is sin." If your heart is in the right place and you are not willfully sinning and committing evil acts then there is no sin. To simply believe something wrong, but you are not aware that it is wrong, and genuinely believe it to be true -- there is no sin.
There are deeper levels of this which others may try to use to refute my conclusion here because they will point to Revelation where it says "all those who make and love lies both have their part in the lake of fire." This does not refute what I said, but demonstrates the importance of seeking the pure Truth in all things out of a sincere and humble heart. But many, like all atheists, assume things without ever having truly sought the truth. They look for excuses and reasons to justify their wicked lives and therefore are guilty of making and loving lies. An atheist is always guilty of pride, bitterness, and a haughty heart. It takes humility and sincerity to receive any real truth in this world. But pride causes people to jump to conclusions before having gathered as much information as possible.