If you ever see the word "sin" mentioned, chances are it's in the context of people doing things wrong, like murder, sleeping around, stealing, sleeping around, eating chocolate cake, sleeping around, etc. I disagree.
Biblically, people don't go to hell because they kill other people. They don't go to hell because they sleep around. They don't go to hell because they eat chocolate cake. Biblically, people go to hell because of how they act towards God.
Look at this bit from Romans 1 (NIV)
18The wrath of God is being revealed from heaven against all the godlessness and wickedness of men who suppress the truth by their wickedness, 19since what may be known about God is plain to them, because God has made it plain to them. 20For since the creation of the world God's invisible qualities—his eternal power and divine nature—have been clearly seen, being understood from what has been made, so that men are without excuse.
21For although they knew God, they neither glorified him as God nor gave thanks to him, but their thinking became futile and their foolish hearts were darkened. 22Although they claimed to be wise, they became fools 23and exchanged the glory of the immortal God for images made to look like mortal man and birds and animals and reptiles.
It then goes on to talk about how their rejection of God works out in all kind of problems, many of which are to do with sex. But they are only symptoms of the real problem.
People don't go to hell because they kill, or because they sleep around, just the same as people don't die because of a cough. People might die because of what causes the cough, and people might go to hell because of what causes them to sleep around.
So what is the real problem?
verse 21 - "For although they knew God, they neither glorified him as God, nor gave thanks to him"
The real problem, and the essence of sin is that we don't thank God enough, and that we don't treat him as if he is God - we don't glorify him.
That is what sin is, and we all do it all the time.
So any questions involving "sinless atheists" or "sinless Muslims" are rubbish, because sin is about not treating God as God. We all do it.