The moral argument states that moral laws require a moral Legislator. Thus, for an atheist, there can be no such thing as objective moral values because they have removed the only objective source of morality from their worldview. If God does not exist, morality also cannot exist. It becomes a matter of opinion. For example, why is murder wrong?
In an effort to get around this, atheists try to present a number of alternative solutions. These often commit the appeal to consequence fallacy. For example, murder is wrong because murderers are held accountable, either by the government or by society.
If they are caught. But this is not always the case, is it? Many murderers got away with their crimes. A famous example is Jack the Ripper. Jack the Ripper was a 19th century serial killer who, to this day, is unidentified. Of course, by this time, s/he will be long dead. Even if we discover the identity of Jack the Ripper, s/he will never stand trial, never spend a minute in prison, Jack the Ripper got away with murder.
So, there are two possibilities. The first, which I'm sure most people will agree is absurd, is that murder is somehow acceptable if you can get away with it. The second is that society and the law are not moral sources. Society and the law can both fail to hold people accountable.
But God can't. Whoever the Ripper was, they have faced God, and every sin they have ever committed has been taken into account. While I cannot say for sure the Ripper never repented, it is highly unlikely (otherwise, s/he'd have turned him/herself in), and so an eternity in Hell awaits him/her.
I'm willing to bet most of my readers have never killed anyone. But murder isn't the only sin. There are many ways we sin against God, and His word clearly tells us there is only one adequate punishment: Death. An eternity in Hell awaits all sinners, not just murderers, and we are all sinners.
But there is hope, because as much as God must hold sinners accountable, He doesn't want a single sinner to perish. Even the Ripper had a chance to repent and be saved. This is because 2,000 years ago, Jesus walked the earth as a man. Unlike the rest of us, He never sinned, and so He deserved no punishment. He received punishment, however. Yours. If you confess Him as Lord and believe He rose from the dead, you can avoid joining the Ripper in Hell, instead inheriting a place in the same Kingdom s/he could have joined you in.