Atheists have a wide variety of views on morality. Some, and I dare say maybe even most, are generally quite good people (by human standards). But you also have those who think it's ok to demolish a monument with a car because the Supreme Court ruled monument can stay up. There are still more sick people who, fully knowing the truth about abortion, support it to the point where it's seen as ok to roundhouse kick a teenage girl for holding up a pro-life sign. Other atheists will happily pick up a gun and shoot people who just want to praise God in their church. Further up the scale, you have Nazis, who committed genocide in ways the world will never forget. Up one more level, you have Communists who killed many more people in ways the world did forget. With the "good without God" mantra going strong, what do you say to those atheists who do blatantly evil things that even their fellow atheists would find abhorrent?
If you're a Christian, you already know the answer. One day, all people, regardless of their personal beliefs, will have to answer to the God who actually created them. Every failure to do what we were created for will be punished, whether Jesus receives that punishment at the cross for Christians, or whether it is received by the sinner themselves in Hell.
But try answering that question as an atheist. Why are evil things objectively evil? The key word there is "objectively". There is a difference between "it's wrong in my opinion" and "it's actually wrong". In this article, I will examine a few of the ways atheists attempt to justify morality within their worldview, and demonstrate that, lacking the Divine Legislator, they have no real basis for morality.
The pain principle
Pain and evil do not always correspond. If I stab a quadriplegic, he won't feel it. It's still evil to kill him. If I go to the doctor, I might feel pain at the treatment, but he's actually doing a good thing. Punishment often hurts, but punishment is, by definition, the appropriate response to evil. And if pain is the standard of evil, murder would be the ultimate good. Murder, especially if there is no afterlife, ends all the victim's pain, whereas saving someone's life may involve pain, as well as increase their potential to feel more pain in the future. Even the creation of life would be an abomination, as not only is the act of childbirth usually quite painful for the mother, but the resulting child is a brand new pain receptor who will inevitably suffer during their life, right up until their unavoidable death. Thus, pain cannot logically be a measure for evil, as being consistent with this would result in a complete reversal of the known moral order.
Because I don't like them.
This statement, and all variations of it, are subjective, and cannot be universally applied. Arguing over personal opinions is fruitless. Atheists actually admit this when they share silly memes about how "if gays can't get married because of your religion, you can't have a cookie because I'm on a diet" and how there's a difference between "I can't do that because of my religion" and "you can't do that because of my religion." Only if there's an objective moral standard that applies to all people, at all times, regardless of their personal beliefs and desires, can you truly say "that was good" or "that was evil". Arguing that someone else's actions are evil because you don't like them is like saying no one can drink cranberry juice because I prefer lemonade.
Most people/society agree they're evil.
This is really just another variation of "I don't like it", but there are a few other problems. First off, you need to define your sample group. Which society? Because I guarantee you, if an atheist travels from England to Saudi Arabia, they're going to find a few differences of opinion. And globally, atheism is in the minority, with most people believing it's wrong to be an atheist, so anyone using this argument should stop being an atheist for consistency's sake.
Second, which time period? Society today is very different from society of the past, and what we do here in the present can drastically alter the future. If that is the case, society is a horrible standard for morality because it just won't sit still. What's wrong today might be right tomorrow, and vice versa. And of course, atheists agree, because there are many things in society they don't like. You'll see a variety of atheists arguing for a variety of counter-cultural things.
Third, what if society is brainwashed? Like it or not, this happens. Every society is subject to false influence. Almost the entire Middle East is dominated by people who think you shouldn't be allowed to question the teachings of Muhammad, for example.
And what about when "most" is defined as 51%? 49% of people are evil because they disagree with the other 51%? Even if we increase what is required for a majority, you have two problems. You're not going to get a much higher level of agreement, nor is it really logical to say that even 1% of people are evil if they disagree with 99%. Why? Simply because truth is not a Democracy! If 99% of people said 1 + 1 = 5, then a maximum of 1% of people could be correct. Therefore, appealing to society is a terrible measure of morality.
Because they're illegal
Again, in which country/era? It was illegal to protect Jews in Nazi Germany. It's illegal for a woman to not wear a burqa in Iran. It's illegal for a native born Malaysian citizen to leave Islam. Fancy obeying these laws? In your own country, you must obey the laws you don't agree with, too. What happens if you don't? You're punished by the brute strength of law enforcement. Therein lies the problem. It's not about who is right, but who has the biggest gun. I don't especially want to live in a world where "you have to kill that Jew, it's the law" is considered a valid argument. Neither should you.
There are consequences for doing them
Not necessarily. Even a country with a perfect legal system, if such could ever exist, would suffer from problems in enforcement. No country has ever had such a perfect system, but there are still good laws in most of them. Yet, those good laws can still be violated, and those who violate them can escape. Jack the Ripper, for example, is long dead, yet we still don't know who s/he was. If atheism was true, Jack the Ripper would have gotten away with murder. Therefore, consequences cannot be a logical standard of morality.
It's how we evolved
Coming up with some bogus story about how morality evolved doesn't overcome the obvious barrier that subjective morality is still subjective morality. "You can't do that because I've evolved to think it's wrong" could justifiably be met with the sarcastic response of "I've evolved to think it's right". On top of that, in order to use Evolution to justify morality, one must first assume Evolution is true, or even remotely valid, which of course it is not. You can no more convince a non-Evolutionist to follow Evolutionary morality, which of course does not exist, than one can convince a non-Christian to follow Christian morality.
We have benefited from following these moral systems
It is ironic that some people posit selfishness as the foundation for non-selfish moral values, and it really highlights the selfish heart of individuals who argue this way. Selfishness, however, could not create selfless moral values such as "love your neighbor as you love yourself", or "love your enemies, pray for those who spitefully use you". Even "do unto others as you would have them do to you" (as opposed to the more common secular version "don't do to others what you wouldn't want them to do to you") is inherently non-selfish, and thus selfishness cannot be its foundation.
Furthermore, "we have benefited from it" is actually an excellent argument for the Holocaust, because we have benefited from some very unethical medical experiments performed on Jews against their wills. Sadly, this argument is particularly persuasive in the modern day. Many people, particularly students, are starting to question whether the Holocaust was actually evil, given that we have benefited from it. Thus, if anything, this particular standard is proof that atheism erodes morality, not that it provides us a foundation for it.
Animals behave this way...
I'm more than willing to admit I've never heard this argument in favor of any behavior except homosexuality, which should tell you just how valid it is even in the eyes of an atheist. [Post publication update: I have since heard a single atheist claim we should look after the elderly because elephants do likewise.] Nevertheless, a consistent atheist who uses it in this case may bring it up in others. If this happens, remember that "you're acting like an animal" is supposed to be an insult. To me, especially as an ex-gay, this just seems to make the argument ironically homophobic. Furthermore, if we apply this logic, society will instantly collapse, as while such brutality "works" in the animal kingdom, human beings simply are not designed for the savagery seen in nature. We're not supposed to be rapists, pedophiles, murderers, thieves etc. We prosecute such crimes precisely because we are better than animals. Therefore, animals are a terrible model for morality.
Conclusion
In spite of all their excuses, atheism has no objective basis for morality. It's all opinion vs. opinion, and the opinion that wins is the one that can be enforced by brute strength. In other words, in atheism, the one who is right is the one with the biggest gun. Now that we've established that atheism leads to the absurd conclusion that good and evil don't exist, let's talk about Jesus.