Without God, atheists have no reason to behavior morally. What's the point of being moral is there is no God?
The idea that atheists have no reason to be moral without a god or religion may be the most popular and repeated myth about atheism out there. It comes up in a variety of forms, but all of them are based on the assumption that the only valid source of morality is a theistic religion, preferably the religion of the speaker which is usually Christianity. Thus without Christianity, people cannot live moral lives. This is supposed to be a reason reject atheism and convert to Christianity.
First, it must be noted that there is no logical connection between this argument's premises and conclusion — it's not a valid argument. Even if we accept that it's true that there is no point in being moral if there is no God, this wouldn't be an argument against atheism in the sense of showing that atheism isn't true, rational, or justified. It wouldn't provide any reason to think that theism generally or Christianity in particular is likely true. It is logically possible that there is no God and that we have no good reasons to behave morally. At most this is a pragmatic reason to adopt some theistic religion, but we'd be doing so on the basis of its supposed usefulness, not because we think it's really true, and this would be contrary to what theistic religions normally teach.
Human Suffering & Morality
There is also a serious but rarely noted problem with this myth in that it assumes that it doesn't matter that more people are happy and fewer people suffer if God does not exist. Consider that carefully for a moment: this myth can only be espoused by someone who doesn't consider either their happiness or their suffering to be especially important unless their god tells them to care. If you are happy, they don't necessarily care. If you suffer, they don't necessarily care. All that matters is whether that happiness or that suffering occurs in the context of the existence of their God or not. If it does, then presumably that happiness and that suffering serve some purpose and so that's OK — otherwise, they're irrelevant.
If a person only refrains from killing because they believe they are so ordered, and the suffering that murder would cause is irrelevant, then what happens when that person starts to think that they have new orders to actually go out and kill? Because the suffering of the victims was never a dispositive issue, what would stop them? This strikes me as an indication that a person is sociopathic. It is, after all, a key characteristic of sociopaths that they are unable to empathize with the feelings of others and, hence, aren't especially concerned if others suffer. I not only reject the assumption that God is necessary to making morality relevant as being illogical, I also reject the implication that the happiness and suffering of others isn't very important as being immoral itself.
Theism & Morality
Now religious theists are certainly entitled to insist that, without orders, they have no good reason to refrain from rape and murder or to help people in need — if the actual suffering of others is completely irrelevant to them, then we should all hope that they continue to believe that they are receiving divine orders to be "good." However irrational or unfounded theism may be, it's preferable that people hold on to these beliefs than that they go around acting on their genuine and sociopathic attitudes. The rest of us, however, are under no obligation to accept the same premises as they — and it probably wouldn't be a good idea to try. If the rest of us are able to behave morally without orders or threats from gods, then we should continue to do so and not be dragged down to others' level.
Morally speaking, it really shouldn't matter whether any gods exist or not — the happiness and suffering of others should play an important role in our decision making either way. The existence of this or that god could, in theory, also have an impact upon our decisions — it all really depends upon how this "god" is defined. When you get right down to it, though, the existence of a god can't make it right to cause people suffering or make it wrong to cause people to be more happy. If a person is not a sociopath and is genuinely moral, such that the happiness and suffering of others really matters to them, then neither the presence nor absence of any gods will fundamentally change anything for them in terms of moral decisions.
The Point of Morality?
So what's the point of being moral if God doesn't exist? It's the same "point" that people should acknowledge if God does exist: because the happiness and suffering of other human beings matter to us such that we should seek, whenever possible, to increase their happiness and decrease their suffering. It's also the "point" that morality is required for human social structures and human communities to survive at all. Neither the presence nor the absence of any gods can change this, and while religious theists may find that their beliefs impact their moral decisions, they cannot claim that their beliefs are prerequisites for making any moral decisions at all.