A common argument from atheists is the idea that one does not have to believe in any god in order to be a good person. And on the surface, that is a true statement, provided the society the atheist finds themself in has a definition of what is morally “good”. Nearly every atheist I’ve ever met who say they are a good person uses murder as an example of something bad they would never do. Which is all well and good seeing that most societies today condemn “murder”. But not all of these societies condemn individuals intentionally killing other individuals (which is the very definition of murder).
Take ‘honor-killings’ for example. In certain areas of the world, a man is allowed to kill a spouse or child who brings dishonor to their house/family. These “dishonorable” acts may vary from (and are not limited to) simply being seen publicly with the wrong person to getting pregnant outside of marriage. Victims of honor killings are almost always girls and young women, though young boys may also be subject to them as well. In such cases, the honor of the family is more important than the life of the individual, therefore it is “good” for that individual to die in order to preserve the family honor. The “goodness” of honor-killings is permitted and encouraged by societies, cultures, and religions who do not adhere to the Word of God.
In the book of Exodus, God gave Moses ten commandments carved into stone tablets. These commandments, upon which the Jewish law is based, defined ten basic tenets all people must adhere in order for man to live in peace with their neighbors. Exodus 20:13 (KJV) gives us what is perhaps the most widely known commandment: “Thou shalt not kill.” At the time, these Ten Commandments was a radical departure from the common morality of ancient societies of the time. Murder was only a crime if those in power in society decided it was a crime. Which often meant the law was subjective to who the murderer was or knew. Here, God says murder is a crime (aka sin) regardless of who you are.
This is the difference between subjective morality and objective morality.
Throughout the span of human history, no society has ever arisen with atheistic beliefs. Ancient Rome, Egypt, Media-Persia, Babylon, and even ancient eastern empires all were based on some form of pantheistic religion which provided a common moral framework upon which their society grew. Each society defined what is good and what is bad.
Atheism is a modern theological phenomenon with only one defining tenet: “There is no god.” Since there is no ultimate being in the atheist’s mind to act as arbiter, judge, or intercessor, atheists are ‘free’ to believe in anything the individual chooses. The only limit to an atheist is whatever they can individually tolerate. So when one atheist says “murder is bad” and another atheist says “murder is okay”, they are both right… in their own minds. A civil society cannot be built among individuals who can each choose their own morality.
So when an atheist claims they can not be believe in God and still be a good person, they are, at best, basing their judgement of what is good on the subjective morality of the society in which they live. Since most atheists making this argument live in the nominally Christian West, its all well and good. But they are not basing their moral judgement on the objective convictions of their faith, but upon the subjective judgement of their surroundings.