Christians,why do you consider people who are non-Christians as lost?

Firstly believing oneself to be right is not unique to Christianity. Most people would not be involved with their religion if they thought they were wrong. And atheists wouldn't be atheists if they thought they weren't right and others wrong. So what is the problem? Most people I know are honest enough to admit they have sinned and committed acts that violate their principles. Yet none of the religions that I am aware of offer to cleanse a person from the stain of that sin. So if a person is still in their sins they are considered lost. Again I don't see why this is a difficult concept to grasp unless the person is unwilling to turn from their sinful behaviour.