Morality without God
Some Christians I speak to are convinced that morality cannot exist without God. Or, alternatively are convinced that the existence of morality proves the existence of God. They think that the alternative to belief in God is moral nihilism. I get the impression that they think a country full of atheists would instantly disintegrate into anarchy as everyone would go out and murder, riot, steal and so forth... So here's a few little exercises to show why all of the above ideas are completely misguided.
Imagine you are a person who does not believe in God. If you were such a person, would you think that murder is an act that harms people? Of course you would. Would you be happy about the idea of someone murdering you or one of your family or a friend? Of course not. Since you care about yourself and various other people in your life, you have a desire that no harm comes to them because you value their well-being.
So you'd want to prevent people killing them. Indeed, you'd want to stop people from causing any sort of serious harm to those you care about. If you were such a person, would you prefer to live in a society that permitted murder and where hundreds of people were murdered every day, or one where murder was illegal and a relatively rare thing? Obviously, you'd prefer to live in a society where murder was rare because you don't want those you care for to be harmed.
In other words you would view murder as a bad thing because it causes harm to those whose well-being you value, and you would want your society to prohibit and condemn murder by making it illegal and discourage it. The same applies for any other behavior you would consider harmful to those you value. Conversely any behaviors you deem beneficial to those you value, you would want your society to encourage and applaud.
Each atheist in society would be like you - being against behaviors that cause harm to people and being supportive of behaviors that are beneficial to people, because they in turn have those whose well-being they care about. All these people would want to see their society use its laws and customs to discourage, condemn and prohibit conduct that harms others and to encourage, praise and allow conduct that helps others. This atheist society would label many things "right" and allow them and label many other things "wrong" and disallow them. They would have a system of morality and do so without believing in God.