The moral argument states that if there is no God, there is no such thing as morality, and thus every time an atheist makes a moral statement, they are borrowing from God. The standard example is murder. Most people recognise that murder is evil, yet the only way murder can truly be evil is if some kind of god has declared it so. A Christian can confidently say, with absolute certainty, that murder is evil, because the Bible, which we believe to be the word of God, forbids murder. An atheist, by contrast, can never consistently say murder is evil.
One way atheists try to disarm this argument is by turning it into a personal attack on the one making it. They make out that the Christian has effectively just confessed that the only thing stopping them from committing murder is if they believe in God. But this is not the argument. It has never been about individual desires, but about consistency, and how atheism lacks it.
All atheists have moral desires. Yes, many of them are opposed to various kinds of murder. But can they say murder is absolutely wrong, or just that they don't like the type of murder they're thinking of at that moment?
Perhaps you think that's a strange way to put it. And the truth is, it is. But the reason I have put it this way is because while "murder" tends to put the image of a shooting or a stabbing or something like that into our heads, atheists are often in favor of other kinds of murder, most commonly abortion.
But why limit this to just humans? Although some atheists like to go the full 10 miles and say all killing is wrong (which itself is an impossible standard, as even farming fruit and vegetables often involves animal casualties as collateral), most atheists are quite happy to eat the deceased corpse of what was otherwise a healthy and happy animal. I have never heard an atheist explain why they believe their life is more valuable than a chicken's, and I've even heard several atheists flat out admit that, in their philosophy, it isn't.
And so rather than revealing the depravity in the believer's heart, the atheist can really only reveal the depravity in his own. No atheist can say murder is absolutely wrong, they can only say they, personally, don't like it. But what if you do? In fact, let's go all the way back to the beginning, but this time, we're going to pretend the atheist didn't make a 10ft straw man. We're going to pretend that the Christian really did say that the only reason he doesn't murder is that he is a Christian. What can the atheist say to him to show that murder is wrong even if there was no God? Answer: Zip. I challenge anyone, from any planet, to explain why, if there was no God, murder would continue to be absolutely wrong rather than just a matter of personal preference. It can't be done.
And so the atheist has three choices. The first ought to make any sane person cringe: They can say out loud that the Holocaust was not a bad thing. The second is to recognise that he is cherry picking his preferred elements from a religious, most likely Christian worldview. The third is none of the above. The "great" thing about moral relativism is that you cannot even condemn hypocrisy. But you can, but you can't, but you can, but you can't, but you can, but you can't, if there are no moral absolutes, you can afford to be inconsistent. You can make up your own morals, and ultimately, the one who is "right" is the one with the biggest gun.
As I said, this ought to offend any sane person. The wonderful thing about God is that He didn't just drop a person in one place, moral absolutes in another and hope they find each other. No, God has written His law on the very hearts of men (Romans 2:14-15). Basic morality is an instinct, and when we have the added bonus of God's revelations, we start to see not only that there are moral absolutes, but that we have broken them in many ways. And this is actually why some people will go as far as to take option one: Say Hitler wasn't necessarily wrong. It's easier to deny moral absolutes than to confess that you are absolutely immoral.
But there is an alternative. The same God that wrote the moral laws we have violated also provided a way to atone for those violations. Jesus Christ, the Son of God, came to earth, lived a perfect life as a man, and died on the cross. In His death, Jesus took your punishment for sin, allowing God to completely dismiss all charges against you. By faith in the resurrection of the Lord Jesus Christ, we can inherit eternal life with God in His perfect Kingdom.
Recommended song: Wolves at the gate - Pulled from the Deep