Myth:
You must think that you are so sinful that God would find you unacceptable.
Response:
This myth is likely to come from a Christian, particularly a Protestant. The idea that all human beings are depraved sinners who know that they are depraved sinners, unworthy of forgiveness or eternal life, has long been an important aspect of Christian doctrine. God, however, is loving and generous enough to forgive us despite how horrible we are. If a Protestant Christian sees this as central to their own life, they may assume that it's central to others' lives as well — even atheists who don't believe in sin or gods.
Not only do atheists not accept any of this, but atheists don't even accept the definition of "sin" used in such a description, so it isn't likely that the claim "You think that you are so sinful that God would find you unacceptable" will be believed, either. Atheists certainly acknowledge that they aren't perfect and that they sometimes do wrong things, but none of them would find the above description to be even remotely accurate.
Atheists do not believe in gods for a wide variety of reasons, but the fear that they wouldn't find approval with some particular god is so unlikely a reason that it doesn't really merit consideration. Such a position almost assumes that the person really does believe in a god but is afraid to admit it — a related, but nevertheless separate myth which is commonly said about atheists. A person who does believe in a god but is afraid to admit it isn't really an atheist. They are a fearful theist, perhaps, but they aren't an atheist because atheists lack belief in the existence of any gods.
Although there are a wide variety of popular myths about atheism and atheists, there is one common theme which comes up repeatedly: the person repeating the myth presumes to know atheists extremely well, even to the point of making claims about their inner lives and experiences without ever having met the atheists in question. This level of arrogance is distressing because it's characteristic of someone who is more interested in their own assumptions and prejudices than in learning anything new about the people and world around them.
When you see the above, more likely than not you are encountering a person who is so wrapped up in their own theological universe that they simply have not figured out that there are others who don't accept the same premises and beliefs. It will be very difficult to have a conversation with them unless they learn how to step outside of their preconceptions about others. Until then, it's unlikely that you will get them to understand that you aren't exactly like they assume you must be.
If you are going to get anywhere with such a person (which may or may not be possible), you'll need to patiently explain to them that you don't share their assumptions about sin, the supernatural, and related matters. Because you don't share these assumptions, it's unreasonable for them to expect you to also share their conclusions about sinners needing a god or salvation. If you are fortunate, they will "get it" and a more productive conversation can ensue. If not, you're probably wasting your time.
