The moral argument is one of the most powerful arguments against atheism, yet it is simultaneously the most tedious to use. Whenever atheists respond to it, it is inevitable they will commit the straw man fallacy. Most commonly, "I can be good without God". We don't need God to tell us to be good, they reason. We do enough good on our own.
This has never been the moral argument. No one says you need God to tell you to do good things, and if you don't believe in Him you'll turn into some sort of soulless psychopath. It does run the risk of reprobation, and we are seeing that in our post-Christian society, but it's true. God has written the law on our hearts, and so even Gentiles, lacking God's law, do by nature the things in the law.
Of course, we also do, by nature, things that are against the law. And this is something atheists conveniently overlook when they make this argument. If we can be considered "good without God" because we happen to do good things without His express command, then can we not also be considered bad without Satan, since we all sin?
The moral argument is simply this: Without some higher authority, there is no good and evil. You cannot make a subjective mistake, you cannot break a law that does not exist, and even if a law exists, it requires a legitimate authority to make it. But atheism cannot provide such an authority. Thus, atheists are stuck between a rock and a hard place, being forced to conclude all actions are equally neutral, or to appeal to a higher power they don't believe exists.
But as a Christian, I do. I believe there is an authority higher than myself, to whom all human beings are accountable. Therefore, I can say that no atheist is good without God. In fact, I can say no one is good but God, because His word says as much. And I can show a few ways we are not good.
Of course, I say we precisely because it isn't an atheist problem. It's a human problem. All of God's people are evil. When God came to Job, he repented, saying "“I have heard of You by the hearing of the ear, But now my eye sees You. Therefore I abhor myself, And repent in dust and ashes.”" (Job 42:5-6). When He came to Isaiah, he lamented "“Woe is me, for I am undone! Because I am a man of unclean lips, And I dwell in the midst of a people of unclean lips; For my eyes have seen the King, The Lord of hosts.”" (Isaiah 6:5). Even Peter, when Jesus came to him as a humble carpenter, fell at His knees and asked "Depart from me, for I am a sinful man, O Lord!" (Luke 5:8).
I do not imagine I would stand before that same God and say anything other. I abhor myself, and repent in ashes. I am a man of unclean lips, and dwell in the midst of a people of unclean lips. I am a sinful man. And not even the proudest of atheists, when they behold that same God, will stand. Were it not so tragic, it would be hilarious to watch blasphemers like Matt Dillahunty claiming he will go to Hell knowing he is morally superior than the thug that sent him there. In truth, if, God forbid, he should go to Hell, he will weep in shame, along with every other miserable soul who passed up the gift of salvation.
But lacking knowledge, atheists still see Hell as unfair. And it's tempting to show why it isn't, just as it is tempting to show how everything else atheists object to, of course assuming they have taken it in context, is also good. And in one sense, that's ok. It can be helpful to show why God was right to wipe out Sodom and Gomorrah, why God was right to prescribe the death penalty for certain sins in Israel, why God is right to condemn sexual immorality etc. But who am I?
Who am I, a mere human being, to stand in judgement over God? Who am I, with no authority, to justify God? Who am I, a sinner, to even say "God is good, this is why"? I have no right to do these things. It's not my place to defend God, and I could no more defend Him than I could defend a lion against a guinea pig.
And a man can no more stand against His judgments than a guinea pig could stand against a lion. Tell me, my atheist friends, what makes you "good without God"? What makes you good at all? What standard do you use? The laws of your land, made by men who are your equals and will often disagree with you? The ever-changing whims of your culture, which clashes with other cultures both within your borders and across the world? Your own futile opinions, formed and maintained in ignorance, and subject to your own subjective desires? None of these things, no standard you can invent or appeal to, will make you good in the eyes of God. And if you're not good in the eyes of your Creator, you're not good at all.
But the good news is when God designed the very laws that condemn us, He included a way to save us. The fancy term is "penal substitutionary atonement". In short, it means while we are sinners, God can make us righteous by punishing Jesus in our place. From His conception to this very day, Jesus perfectly obeyed God, and so He is good. Yet, He died as if He was bad. He died like a sinner. Like the sinner you are. Because He took your punishment, He can give you His reward. The sole condition for this is faith. Those who confess Jesus as Lord, and believe in their heart God raised Him from the dead, are forgiven of their sins, and will inherit eternal life in the Kingdom of Heaven. You can try being good on your own, but you already failed. This is the one and only way to be saved.