Myth:
By your belief, you are just made of atoms and nothing more, so you have no reason to care about others.
Response:
This myth may appear to be incoherent — what do atoms have to do with caring about others? — but it's an expression of a popular theistic argument against atheistic materialism. Religious theists believe that "immaterial" feelings like love cannot have a material basis and must, instead, come from our immaterial souls which are created by an immaterial God. If someone doesn't believe that such immaterial beings are real, then they must not believe that immaterial feelings like love are real.
Atheists don't typically believe in souls; instead, they usually adopt a materialist approach to the question of life and argue that who we are is ultimately dependent upon our physical bodies. We are what we see, with no souls to live on after our bodies die. We are, then, just "atoms." These views are not universal among atheists and they aren't required by atheism, but they are the most common views among skeptical, scientific atheists in the West, so it's fair to focus on them here.
For Christians, on the other hand, our essential nature can be found not in our physical bodies, but rather in our immaterial souls. This is presumably why they and man other theists think that a person should care about others: because the person doing the caring has a soul and/or because those being cared for have souls. Is either position reasonable? I don't think so.
In the first place, there is no sound basis to think that the capacity to care for others is in any way dependent upon having a soul. Caring, like any emotion, is adequately explained by reference to the physical, material brain. Secondly, there isn't any good reason to think that people would be more worthy of love, kindness, or consideration because of having souls. To approach it from the other direction, there doesn't seem to be any reason to think that people would be less worthy (or not worthy at all) of love, kindness, or consideration if they didn't have souls.
Anyone familiar with logical fallacies will immediately recognize the Fallacy of Composition in the above myth: just because individual pieces of matter cannot experience emotions like love, this doesn't mean that all things made out of matter (like humans) also cannot love. Once we recognize this as a fallacy, it becomes easier to understand that why the argument fails and thus also why materialism doesn't restrict us to accepting only the existence of properties that can be found in atoms.
Instead, we can also accept the existence of properties that only appear in certain configurations of matter — emergent properties that occur due to how different types of matter and energy interact. That in turn allows for the existence of things like thought and emotions which are not themselves obviously physical.
It's doubtful that many religious theists, including those who espouse myths like this, really believe it themselves because they don't behave as though it were true. Many (if not most) Christians don't think that other animals have souls (it doesn't appear to be an orthodox Christian belief), but that doesn't stop them from caring about animals — even caring very, very deeply. Why? I would argue that it is because animals manifest a capacity both for suffering and for happiness (no, not all animals — but quite a few and especially those animals which we take into our lives, like pets). This alone is sufficient to arouse in humans feelings of empathy, happiness, and sadness.
Why should other human beings be treated with any less consideration? If you can care for and love a cat or dog on the basis of its capacity for suffering and happiness, why not a human being? Here we can see how and why one should care about others even if they are "just made of atoms": they can feel and they can suffer. That should be sufficient, even for a Christian who believes that he should "do unto others" as he would have them do unto him.
Sometimes this myth about atheists is not offered in all seriousness — the person doesn't really believe that atheists can't care for others. Instead, the person is trying to show that because atheists do care for others, then atheists believe in "non-material" things like love and is thus not really a materialist after all. Once materialism is abandoned, then presumably all sorts of "spiritual" things like souls and gods might find their way in. The theist can be disabused of this notion by correcting them and pointing out the fallacy underlying their entire argument.
