Atheists have probably never heard an intelligent presentation of our Christian beliefs; if they did, they would convert and become Christians themselves.
Christians who are completely caught up in their religious beliefs sometimes have trouble understanding why someone else would fail to agree with them. They regard their religion, doctrines, and traditions as so obviously true that it's inconceivable that a person could take a serious look at them without ultimately accepting them. Assuming that the atheist isn't an idiot or being perverse, then the most viable conclusion to draw from their disbelief is that they are ignorant of the facts.
Sometimes, this ignorance is assumed to be a complete lack of familiarity with Christianity and Christian teachings. Some Christians continue to treat the world as if the Christian message were still new and that there are large, ignorant masses out there who must be evangelized to. More seem to recognize, though, that there aren't many people left who have heard of Christianity and who haven't been exposed to its basic message. This means that persistent rejection of Christianity must be attributed to some defect in the presentation of the message — it certainly can't be because the message itself can be deemed unworthy of belief.
Given that so many atheists who reject Christianity do so on intellectual grounds, saying that the arguments on behalf of Christianity are unsound and unpersuasive, then it must be that the presentations of Christianity were simply not intelligent enough. The apologists who tried to evangelize atheists must have failed to provide the strongest, most intelligent arguments available and thus failed to convince the atheists that Christianity is a sound, rational, defensible religion to adopt.
The first problem with this has already been brought up: it's based on the assumption that there couldn't be anything wrong with the Christian message itself and thus also that the Christians couldn't possibly be wrong — or that there might be sound reasons for disagreeing with Christianity, even if it's right. There is no support offered for this assumption, it's just taken for granted without argument, discussion, or closer consideration.
The second problem is the fact that so many atheists are familiar not just with Christianity and the Christian message, but also with many of the most common arguments offered on behalf of Christianity. It's true that they may not be familiar with the most sophisticated versions of those arguments developed by Christian philosophers and theologians, but then again neither are most Christians themselves. If a person can be a Christian without knowing those arguments, then can't a person be an atheist without knowing them?
This leads us to a third problem: most Christians are not only unfamiliar with the most sophisticated theological arguments on behalf of the existence of God and the truth of Christianity, they aren't even familiar with moderately sophisticated versions of these arguments. Far too often, they barely know the most superficial versions of these arguments and know little or nothing at all about the most common responses, objections, or rebuttals.
Atheists, on the other hand, frequently understand more about these arguments and their rebuttals than the Christian apologist who is trying to evangelize them. This isn't true of all atheists, naturally, but it is true of many — and it only needs to be true of a few for the above myth to be undermined. The problem thus isn't that atheists have never encountered an intelligent presentation of Christianity, it's that they don't consider even intelligent presentations of Christianity do provide compelling, credible reasons to think that Christianity is true.