When theists say that atheism cannot exist with morality, secular humanists are quick to say that it is a misconception about atheism. But their objection is misplaced. This isn't so much a misconception about atheism as it is about atheists.
Yes, there are some theists who do indeed try to assert that atheists can't be moral, and secular humanists are right to object, because such a claim is not true. Atheists can in fact be ethical.
However, while it is true that atheists can be moral, it is NOT true that one can be logically consistent and be an atheist and a moral realist at the same time. This latter claim is the true argument, not the former.
Now, as to why one cannot be an atheist and be a moral realist at the same time, there are two very similar reasons why this is so, the first coming from David Hume, one of the greatest philosophers to ever live, and the second coming from J.L. Mackie.
First the Humean argument. David Hume famously pointed out that one cannot extract an ought-statement from an is-statement. You cannot get an ought from an is, or in more technical terms, one cannot derive a prescriptive fact from a descriptive fact.
Batman, because he failed to point to a metaphysical, transcendent and ontological foundation, cannot make the case that the joker ought to prefer choice A over choice B. The joker, realizing this, throws Batman's assertion back in his face. The joker laughs in his face, and points out that the secular humanist's rules are a "bad joke." And that the only sensible way to live in his world is without rules. To be a nihilist like himself.
Even with all of his strength, Batman cannot make his humanistic assertion true. In an atheistic worldview, it simply doesn't follow logically.
On an atheistic worldview, the only transcendental facts are descriptive facts. By transcendental I mean facts which are true regardless of what anyone thinks.
Now this being said, there can therefore be no prescriptive facts on an atheistic worldview. Therefore, if atheists are to be consistent, they have to move beyond their short-sighted secular humanism and move into the realm of moral nihilism and sit alongside Ledger's joker.
Now perhaps morality is just a property. Perhaps certain actions, like shoving a pencil into someone's head, has the property of being wrong, and other actions, like saving people from a hospital rigged to blow, has the property of being right.
Forgetting for the moment that morality is not descriptive, but rather is a series of statements of certain actions one ought to do or ought not do, we can turn to J.L. Mackie's take on this.
To say that naturalistic, material objects can stand in a moral relation to one another is absurd. What does it even mean for one object to stand in a moral relationship with another object? It is meaningless.
The ethical skeptic watching this video might feel tempted to ask "Why does God solve the is-ought gap?" and feel as if he won the day. The problem is, that God's commands are not themselves descriptive, but are prescriptive.
Therefore, as an atheist, you must either remain silent like Batman, or speak with cynical greatness like the Joker. Be permitted by Dostoevsky to drink up the sea with Nietzsche.