Its been quite interesting to see how many atheists there are who believe that objective morality exists. Actually, I would say that the majority believe in objective morality. However, objective morality is inconsistent with the atheist worldview; they don't have a foundation for acting in a "moral" way versus an "immoral" way. I'm not saying that atheists can't be moral; they can. I'm just saying that they can't justify it. Here's why.
Morality implies "oughtness". How something ought to behave. That implies that you understand that that thing (that ought to behave in a certain way) was designed to behave in the expected way. Example: A watch ought to keep time. It is designed to keep time; therefore, it ought to. If it were not designed to do anything, it ought (is expected) to do nothing.
Atheism posits that humans and the universe have no design or purpose, period. Therefore, it must be concluded that atheism has no room for moral (among other types of) "oughtness".
Does "oughtness" flow logically from "design" or "expectation"? The atheist might be able to get away from the conclusion above by claiming "oughtness" just implies an expectation. But I would have to question what they base their "expectation" on. If they want to base it on history (rather than design) then, they must determine which parts of history they want to base the expectation on, and I would ask them why they choose those certain parts of history and not others.
Now, some atheists have tried to explain the foundation for their belief in objective morality by pointing to examples in the world. They argue along the lines of "look at society; obviously, murder is wrong" or "obviously, stealing is wrong". They use examples to prove "why". The problem is examples don't prove "why" something is true; they only prove "that" something is true. Atheists still need to provide a reason "why" they ought to act a certain way.
Once again, I'm not saying that atheists can't be moral. I'm saying there is no objective foundation for determining why a certain behavior is moral or immoral in their worldview.
Not only does Christianity explain "why" objective morality exists (it is the very nature of God), but it explains "how" an atheist can be moral, yet believe something completely opposite.
Here's a good article on the subject from Dr. William Lane Craig:
Can We Be Good Without God?
Relativism: Feet Firmly Planted in Mid-Air by Greg Koukl is a good book about atheism and morality.
Here's a video from Greg Koukl. He is asked if pain and suffering disprove God's existence.
