By far one of the most misunderstood arguments for God is the argument from morality. Atheists often assume Christians who use it are arguing that if someone does not believe in God, they are guaranteed to commit evil. That is not the argument (and it is a serious misunderstanding of the Gospel, which ultimately says we're all evil, and need to be saved). Rather, the argument is that without God, you cannot know what good and evil are in the first place.
The Bible never claims that non-Christians cannot be good. In fact, Romans 2:14-15 assumes that even if someone does not know God, they will occasionally obey Him of their own free will, just because their conscience tells them to. But evidently we all have different measures of the law of God written on our hearts. This is proven indisputably by the number of different moral opinions that human beings have between us. No two atheists have exactly the same beliefs. Even Christians differ significantly, particularly when we're more or less involved in Bible study. Which brings us to the question: Who, if anyone, is right? How do we know?
That's where the flags in the above image come in. In the image, we have 3 national flags. The American flag is well known throughout the world. Whether you love it, hate it or just don't care, most of us could pick out the American flag. Few of us will get it wrong, and to my knowledge there are no flags similar to it. What about the other two? The challenge could go either way. I could ask you to tell me which nation they represent, and a lot of people, especially not having read the image, would have no clue. Alternatively, I could ask you to identify the Danish flag, and you'd know which one it isn't, but a lot of you might not know which one it is. If I asked you to distinguish between the Danish and Norwegian flags, you might succeed, you might get them mixed up. All these possibilities have one thing in common: There is an objectively correct standard that you may or may not know.
But how does this relate to morality? Let's throw out three behaviours: Theft, alcoholism and veganism. Each of these behaviours have moral implications. Question one: Is there a way to tell if they are "good" or "evil"? If there is no God, and man is the highest authority, the answer is ultimately no. You'll notice that when atheists try to defend the existence of objective morality, they always appeal to a standard that is ultimately as subjective as preferred ice cream flavor.
But if there is a God, the answer is yes, there is a way to tell if these actions are good or evil. But here's the problem: An atheist who knows that there is an objective standard is just like a person who doesn't have a clue about world flags. You know that there is a standard, but you don't know what it is. But a Christian can. A Christian knows there is an objective standard because there is a God who gives commands and expects us to follow them. More importantly, we know what those commands are, or at least where they can be looked up.
An atheist can take a stab at figuring out right and wrong, and they may even be right the majority of the time (again, God gave us a conscience, so it's not like we're just wondering in the dark without His word), but at the end of the day, a lucky guess is just that. Without God, an atheist can be "good" in the sense that they can obey their God given conscience. But they cannot have an objective sense of morality. Thus, by admitting they know a moral standard exists, they implicitly admit the source of it.