It's not so much either fact or opinion, but the philosophical consequence of removing God. If there's no God or religion, then there's only our individual perspectives of what is right and wrong. In other words, right and wrong are relative to the extent to which we can impose it on others. There can be no appeal to justice because there would no higher authority to which to appeal. The only arbiter for justice would be power; and so life would become defined by the rule of the strong. And on what basis, other than political expediency, could you demand the strong to live according to your principles?
Ah, the infamous "without God, people have no reason to act morally." Before answering your question to me AmericanCatholic, let me pose one to you. Do you only act kindly to others because you are scared that God, the ultimate authority, will punish you if you don't?
Secondly, as Armistead pointed out, atheists have perfectly good reasons to believe that people do not necessarily need God to act morally. The term "social contract" is popular among atheists - which is basically the equivalent of the Golden Rule. If you don't harm me, I won't harm you. Since humans are relatively smart, and have good memories, we tend to hold grudges against people that behave badly. Furthermore, if I act badly, people are likely to not help me out when I need it. Thus, I act nicely to others in the hopes and understanding that others will reciprocate the action. This phenomenon is also documented and well studied in evolutionary biology, and is called reciprocal altruism.
Now, you asked "And on what basis, other than political expediency, could you demand the strong to live according to your principles?" Well, I suppose that reciprocal altruism falls loosely under the category of political expediency. However, this is not the only reason that humans act morally towards each other. Most of us, with the exception of psychopaths, have a sense of empathy. We are able to place ourselves in the situation of others, and thus feel badly for others when they are experiencing troubles.
Thirdly, even ancient societies (yes, before the time of Christ, and even before the time of Moses) realized that societies do not function if people rape/murder/steal from each other. To paraphrase Christopher Hitchens, it is hard to imagine that Moses could have led the slaves out of Egypt and into Israel if they were busy stealing, murdering, and raping each other. Human morals have existed long before Biblical characters had them 'revealed' by God.