The idea is that if right and wrong, good and bad exist at all, then their must be a standard by which they are measured. You can't judge a crooked line if you have no concept of a straight one. (CS Lewis in Mere Christianity.) If you believe in morality, then you are stating your belief in a standard of Truth, an absolute, otherwise there is no absolute standard of morality, thus there is no right and wrong. If all truth is relative, and all morality is relative, then there is no way to judge someone as doing wrong. Most people, including atheists, agnostics, and nonreligious would agree that morally reprehensible things such as genocide, rape, and child abuse are all wrong. But to say that these are wrong is to admit that there is a standard by which they are judged, which is to admit that there is an absolute truth, namely morality.
If there is no absolutes, then none of the aforementioned crimes can be condemned as wrong because to the individual committing them, they wouldn't be considered wrong. I know some will argue that outside of God, morality exists to the extent that you don't cause harm to your neighbor and that such a belief doesn't require a belief in absolutes. I think that the mere statement of that is an absolute otherwise the definition of "harm" could be any number of things the same way without an absolute "right" or "wrong" could be anything. Harm to one person could be different to another.
It seems to me that Lewis and Zacharias are correct (an odd statement considering their intellect is exponentially greater than mine.) The presence of a belief about morality, or such a thing as "good" and "bad" is to admit that their is an absolute standard, and as such something must have created that standard. To be an atheist and believe in morality seems to be a contradiction in philosophies that is irreconcilable.