Myth:
You must be an atheist because you've never heard Jesus or the Gospel (or: Christianity, Islam, God, Muhammad, etc.).
Response:
There seems to be a belief on the part of many theists — especially Christians and some Muslims — that the only reason why a person would not be a member of their religion is because they are simply unfamiliar with it. Or, to put it in Christian terms, because one hasn't heard the "Good News" which they of course now wish to share. As a result, it is common for these believers to immediately launch into preaching in order to rectify that situation.
Of course, it is possible some random atheist really hasn't heard about Jesus or God or Islam, but that is exceedingly unlikely. In fact, it is much more likely that the atheist you are talking to was at one time a Christian, Jew, or Muslim. Although some atheists were raised without belief in any gods, most seem to have started out in a religious household and only later became atheists.
Moreover, in a great many cases it is actually the knowledge of religion which has been a key factor in leading a person to atheism. Many atheists have studied religion extensively, and the more we have learned, the less accepting we have been of what religious authorities have traditionally taught. Our studies have led us to conclude that the scriptures of various religions are not reliable and that theism itself is not reasonable.
So, assuming that the atheist hasn't heard about your religion is unlikely to get you very far — you may even find that the atheist knows more than you! As a matter of fact, there is a good chance that you will only irritate the person and incite a hostile reaction. It's natural for people to feel a bit insulted when someone they don't know suggests that they are completely unfamiliar with a religion which they have studied (and in fact may have once been a devout member of). It's just good manners to refrain from making assumptions about the experience and knowledge of a stranger.
If you want to learn about atheism, you need to ask questions. If you want to learn about a specific atheist, you need to ask specific questions. Making lots of assumptions will only cause people to dismiss you as not being worth an involved conversation — and that is understandable. A person who is interested in learning things won't start out by assuming that they already know everything relevant to the topic.
If it turns out that the atheist you are talking to doesn't know anything about your religion, then you may feel free to share with her some of your beliefs, but a certain amount of caution is in order here. Simply sharing for the purpose of educating and informing is not a problem, assuming that they actually do want to learn more. For many believers, however, "sharing" has the additional connotation of proselytizing.
It is understandable that a person may believe their religion so strongly that they are eager to "share" it with others in fashion designed to convert them to the same beliefs, but it would be arrogant to assume that others would necessarily welcome such sharing. Atheists in particular are frequent targets of conversion efforts and, over time, it becomes rather annoying. A little bit of sensitivity and generosity on such a matter will go a long way — atheists have to listen to proselytization efforts so often that a believer who doesn't try is sometimes a welcome and appreciated change!
