Atheism just a phase you are going through. You'll change your mind soon enough.
This sort of question is asked much more often of young atheists than of adults, perhaps because young people do go through a number of "phases" during which they explore various ideas, philosophies, and positions. Although the term "phase" may be used here in a derogatory manner, it shouldn't be. There is nothing wrong with such exploration and experimentation, so long as it is recognized and accepted as such. If someone is going through an "atheism" phase, what's wrong with that?
On the other hand, the accusatory nature in which this question is often posed must be addressed directly. It assumes that an atheist isn't "serious" about atheism and that an atheist hasn't really given a lot of thought to what atheism (and, by implication, theism and religion as well) means or entails. Indeed, this might actually be true in some cases. Not everyone who calls themselves an atheist has necessarily thought a great deal about what atheism, theism, and religion are all about.
It seems to be assumed that by accusing the atheist of not really having given a lot of thought to atheism, it can be concluded not only that this atheist has rejected theism and/or religion for inadequate reasons, but in fact that no atheist has ever had sufficient grounds for their rejection, thus rendering atheism on the whole unreasonable. The truth of the matter is very different, however.
The first thing to note is that not everyone who is an atheist is "going through a phase." Some might be, but most aren't at least, no more so than the average theist is. In a sense, we may all be "going through phases" because we don't necessarily retain the exact same beliefs and attitudes throughout the whole of our lives. Not only is there nothing wrong with that, we should in fact expect it of mentally healthy adults. The problem lies not with change and development, but with intellectual stagnation. As a result, anything might be a "phase" in our lives, but that doesn't mean we haven't given it a lot of thought.
The second thing to note is that, even if a person is just going through a brief "phase" of atheism and hasn't really given a great deal of thought to the issue, that has absolutely no implications for atheism itself. The same is true, of course, if someone is just going through a "phase" of theism and hasn't really given it much thought. The motivations and circumstances of any particular person does not validate or undermine the things which they believe; ideas must stand or fall on their own because of the arguments which can be used in support of or against them.
Finally, while it is true that some atheists, and especially those who are younger or simply new to atheism, may not have yet given a lot of thought to atheism, theism, and religion, we can't claim that the opposite is true of religious theists. On the contrary, it is often the case that atheists are the ones who have given the most thought and attention to these issues.
Theists are commonly indoctrinated into a particular religion and theistic belief system at a young age and then don't return to reconsider the foundations of their beliefs for a long time if ever. An atheist who has left some form of religious theism, however, has often done so after some serious investigation into the natures of theism and religion. They have reconsidered their prior indoctrination and perhaps know more about the history, origins, and details of their religion than many current believers.