I know a lot of people would disagree with me, but I want to try and respond to this.
At the heart of all morality, all ethics, all human interaction, be it political, economic, relationships, etc, lies our beliefs in what is right and wrong.
It is very hard to define a moral system without a yardstick. There are really two main choices. Either there is no universal yardstick, or there is one. If we assume that one does not exist, then we are in a situation, much like in the past, where a foot was defined by the size of the king/lord/etc foot. How tall you were could change depending on who was in charge. On the other hand, once a universal is found, then we can know how tall we are without reference to who is in charge.
There can be no universal yardstick unless that is from the outside. There can be no absolute right and wrong if our yardstick comes from within... we would each arrive at a different yardstick.
Whether there is such a thing as absolute right and wrong is a very important question for morality, ethics, etc. For most people, absolute morality comes from some concept of God.
How we treat one another and why we do so is intrinsically tied to the question 'is there a God'. Trying to determine if there is a God, where we came from, and how is at the heart of so much.
I am not arguing that one cannot have a moral code without God. I am simply stating that there can be no absolute morality without God. I am not trying to argue one way or the other whether absolute morality is better than relative morality. That conversation is better left to another thread. However, how and why we treat one another the way we do is part and parcel of this conversation.