It means simply there is no such thing (never has been never will be )as an Atheist. What you have is Believers and unbelievers... Atheism is a pure impossibility.... you would have to know everything,that's why the fool has said in his heart there is no God,(what does that make the person who says it outload?) How do we know that there are no atheists? Even agnostics are not physically/mentally blind,even they know there is a God,God has made it CLEAR to them,but their hearts cannot accept this 'sight' so they say that they don't know!! Like the person who refuses to see what is in front of him,BECAUSE he keeps putting his hands over his eyes! It is truly shocking the power of denial,and none more so than denying God's very exsistence.They ARE (not might be) without excuse.Rom 1