The great thing about God's morality is that it isn't arbitrary. There is purpose behind it, and as a result, it is usually possible to make secular arguments defending it. This is beneficial for Christians, because it means we actually have power even beyond the moral argument. Not only can we show that you need God for objective morality to exist, but you can even show how Christian morality works out in the real world. Even many atheists agree, at least in part, with Christian morality, with some of them even admitting that Christianity is an excellent shield against worse things.
But this is a double edged sword. Because it is possible to make a secular case for good morals, atheists like to claim that it is, therefore, possible to do so without God. It's just a matter of cause and effect. Do "good", and it works well. Do "evil", and it doesn't. But this still doesn't solve the problem for atheists, because if morality really is a matter of cause and effect, you have to establish:
1. Which effect is best?
2. Which causes are justifiable?
3. Why?
In Christianity, the answer to these questions is simple: "Let us hear the conclusion of the whole matter: Fear God and keep His commandments, For this is man’s all. For God will bring every work into judgment, Including every secret thing, Whether good or evil." (Ecclesiastes 12:13-14).
From this one verse, we can answer all three questions from a Christian perspective:
1. Desired effect: Right relationship with God.
2. Justifiable causes: Obedience (more on that later).
3. Reason: It's literally the entire reason He created us.
An atheist, however, will struggle to even get an answer to question one. Because they do not believe life was created with a purpose, they believe they can give it their own. But each of them gives it their own! Are you into wrestling? Perhaps you are, perhaps you're not. To Patrick "Rowdy Ricky" Knight, however, wrestling is his purpose in life. I can't find the exact quote, and frankly, I can't be bothered to borrow the movie "Fighting With My Family" from my brother to watch him say it again (go watch the movie yourself), but it was something along the lines of wrestling is "true salvation".
Now, I'm tempted to go more along the lines of "did wrestling go to the cross to pay for your life as a bank robber?", or "when you die, will wrestling attend your hearing at the judgement seat of God?", but setting that simplistic reasoning aside, even people who are into wrestling typically don't make a religion out of it. Patrick Knight is Patrick Knight, and you, my friend, are not (unless, on the off chance, Patrick Knight happens to come across this article, at which point Ricky, simply pretend you are not, and the point I want to make should still land).
Setting aside Ricky Knight, we still see some very major conflicts, even between atheists. Should we all be vegans, for example? Watch Cosmic Sceptic debate Matt Dillahunty on the topic and you'll see my point. Both of these men make a career selling their atheism, yet they are divided on the issue of carnivory. Matt is a carnivore, with his primary goals being his health and pleasure, whereas Cosmic Sceptic (Alex O’Connor) is a vegan. How do these two men differ so radically on their morality? Because they have different goals. And, thanks to their atheism, they have no way to establish which goal is better to pursue.
Now, with regard to animal rights, the Bible isn't as simple as "be vegan" or "eat what you want". Man's dominion over the animals does allow us to eat them, but it comes with the responsibility of caring for them, and not abusing them. It's a long and complicated discussion that would take us away from today's topic, but ultimately, Christians have ground for unity here. We're all reading the same Bible, and so as long as we agree to stick to it (which, sadly, many don't), we will all come to the same conclusions. If we don't, God will actually judge between us. If one Christian wants to enforce veganism, one Christian wants to wipe out entire species, and one Christian wants to obey the Bible, all three can be judged by the same standard, and a maximum of one of them is right. But with Matt and Alex, neither is right or wrong. They can bicker about the effects of their actions, but the question is who cares? Whether or not a calf really can be distressed beyond belief at being separated from its mother is something we can observe and come to an agreement on, but Matt and Alex can debate all day about whether that is justifiable or not. If their goals are different, so are the means by which they can achieve them. If Matt wants bacon, he can justify killing pigs. If Alex wants to enforce veganism, he can justify protecting pigs. Both positions are morally equal if atheism is true.
So already we see that atheists can't pass the first hurdle, but let's act like they can. After all, we can all agree that we want to live healthy lives. So, let's establish the goal of eliminating diseases. Question 2 is which causes are justifiable in order to achieve this effect? There is conflict even here. I'm going to skip over modern times with the different opinions on how to deal with Covid-19 and simply ask this: Did you know there were actually some very useful medical advancements that came out of the Holocaust? Personally, I didn't, until I heard atheists start justifying the Holocaust!
Now, looking at the world today, I'd say we're in poor shape. I honestly never thought I'd live in an era where Joe Biden would be considered a legitimate contender for president of the U.S., yet he currently sits in the White House. But in spite of how far we have fallen as people, I have just enough faith in humanity to believe that most of us still know that the way the Nazis treated the Jews was wrong. Why, then, are so many people capable of justifying it because "medical advancements"? Medical advancements are great, but I can say, without any question or doubt, that human lives are not expendable, and so taking them without due cause is not acceptable, no matter the outcome.
But apparently, some disagree. Indeed, the moral implications of the Holocaust are not the only issues atheists divide on when it comes to the means justified for medical advancements. Now, if morality really is just cause and effect, this should not even be debateable. If the effect we want is medical advancement, any means should be justifiable. The same end is achieved, after all. But if not, then even a good end cannot be legitimately achieved by evil means. If you admit the ends don't justify the means, you admit that morality goes deeper than just cause and effect.
And so for the final question: Why? Why does all of this matter? Why did we select our means, and why did we select the justification? For the atheist, this may be as arbitrary as "because I/we like it this way". And this, honestly, seems to be the most common answer. But is it a feasible one? Hardly. Your opinions were born during your lifetime, and will either die with you, or before you. You can't change reality just by wanting to! And there are no other atheistic standards that are any stronger.
So, atheism can provide no real foundation for morality. It is a parasite that must either leech off Theism, or simply admit that moral absolutes, at least according to atheism, do not exist. By contrast, as I have already demonstrated, Christianity can answer all three questions. The effect is a right relationship with God. The cause is obedience. The reason is God's purpose.
However, you may recall how I said that we would return to obedience. Here's the scary thing: We all fail that. And it's not just a case of we once failed, but can easily succeed. Past failures must be accounted for, but on top of that, they continue. No one is perfect. Jesus, and Jesus alone, walked in perfect obedience. The rest of us have rebelled against God from our youth, and will continue until our inevitable death. The result? Death.
But there is an alternative. Jesus obeyed. Yet, He suffered and died as if He had also rebelled. His innocence, combined with His divinity, makes Him the perfect, and only candidate as our Saviour. When He died, He died for us. When He rose, He rose for us. If we repent of our sins, confess Him as Lord, and believe in our hearts that God raised Him from the dead, we will be saved. The effect of failing this is the second death. No means are justified to that end.