The moral argument is particularly difficult for atheists to respond to, as they actually cannot respond to it without legitimising it. Most attempts to respond to it are actually a straw man. "I don't need God to tell me to be a good person" is a straw man, because the argument is not that without God, you cannot be a good person, but that, without God, "good" is relative. You don't need God to dictate your behaviour. You do the things you think are good, and that's good for you. But there are irreconcilable differences between atheists when it comes to what is good and what is not.
Some atheists like to challenge this by claiming that, even lacking God, you can argue for morality. They argue from cause and effect. If an action causes a desirable outcome, it is good. If it causes an undesirable outcome, it is bad. See? Morality. The problem? The outcome has not been established as good or evil. Using cause and effect, you can judge any action as good or evil. You could as easily live by the mantra "spend no money" as "do no harm". If I donate £20 to a homeless charity, that's good in Christianity, but bad in Richianity. The action is identical, the end result is identical, the moral attribution is not.
And indeed, even the end result is not always the standard, because there are multiple ways to achieve the same outcome. Most of us agree that providing for our own children is good, but how should we go about that? Well, if I get a decent job and work hard at it, my children are provided for. If I become an expert thief, my children are still provided for. One is good, the other is evil.
Of course, this is where the atheist starts pulling out more negative effects of those moral actions. "But stealing hurts other people". Ok, so you're telling me "thou shalt not harm other people", which is still a moral claim. Try again. "You might get caught and that harms you". Ok, then you are telling me "thou shalt not take unnecessary risks". Not to mention you are basically saying if you can do evil secretly, it's no longer evil. Good ol' Jack the Ripper was a stand up guy by that logic.
Do you see, then, how atheism cannot account for morality? There are only two possibilities: Divine law, or spiritual anarchy. Either there is a God, and His laws are absolute, or there is no God, and mankind is stuck in a permanent struggle to enforce our will upon other members of our species. The one who is right is the one with the biggest gun. Put simply, either there is a God, or we are the gods.
Needless to say, that is the root of our problem. The first sin the devil ever committed: "I will be like the Most High". The first lie he ever told to make mankind sin? "You will not die, you will be as God". This has always been the root of our problem. We have rejected God's ways, and turned instead to our own. And unfortunately, we reap what we sow. When you rebel against the Author of life, what consequence is there but death? Thankfully, being the Author of life, God takes no pleasure in the death of the wicked.
Thus, He provided an alternative. 2,000 years ago, God opened the womb of a young woman named Mary; a virgin. She conceived and bore a Son, Jesus, who was God in flesh. Jesus lived a sinless life, and yet He became obedient to death. He died on a cross, whereon He suffered the full penalty for sin. After that, He was laid in a tomb, and on the third day, He rose to life again. Because of this, we who are rebels have been granted a second chance. We, who are guilty, can stand before God and be judged righteous, because Jesus the Righteous was judged as guilty. All it takes is to repent, confess Jesus as Lord, and believe He rose from the dead. Do no harm to yourself: Accept this Gospel.