Some acts are wrong because of their results: If I would be hit by a truck, then this is a bad time to cross the street. But other acts are wrong no matter what their results: No good consequence can make it right to deliberately take innocent human life. Such an act is intrinsically evil.
What makes an act intrinsically evil is that by its very nature it cannot be put in right relationship to our ultimate final end, which is God. If this is the correct explanation, then we should expect atheists – who do not believe that this ultimate final end exists – to have difficulty recognizing that there are any intrinsically evil acts. This is in fact what we find.
Even an atheist who does believe in intrinsically evil acts will have a hard time conceding that such acts are never to be done. If thinks there is no God, he will be more than usually subject to the temptation to play that role himself; the counsel “Do the right thing, and let God take care of the consequences” is useless to him. Instead he will say "Let us do evil so that good will result."
Notice that I am not denying that the atheist has a conscience, which testifies within him to the awful wrong of wrong. But he will have difficulty accounting for his conscience, too. For just as conscience speaks to everyone, so it speaks to him: Not in his own voice, but in the voice of Another -- Someone in whom he has chosen not to believe.