One of the central questions of philosophy is the question "What is the Good?" What does it mean to say that something is good?
One possible answer to this question is that good is that which conduces to happiness and well-being. If that's so, some argue, then one consequence is that God is not needed either for us to know the good or to do it. All we need do is employ our reason to determine what it is that makes human beings happy and then we can build an ethics around that. This is the argument made by naturalists, i.e. those who believe that all that exists is the natural world of material objects and physical forces. The naturalist does not believe that there is any supernature.
There's a crucial difficulty here, though. How can any ethics grounded in human reason obligate me to care about the good of other people? In other words, why would someone be wrong to be an egoist who cares only about his own happiness and well-being?
The whole point of ethics is to provide an answer to this question and to it naturalism has no answer. All the naturalist can do is define what good is. He cannot tell us why we should care about the good of others, except, perhaps, insofar as we benefit from doing so.
But this is just another form of egoism in which one tends to the good of others so as to benefit from it oneself. If there's no benefit to me in helping others, for example by contributing money to a third-world relief fund, it wouldn't be wrong to selfishly withhold my assistance. If there's much benefit to me in employing slave labor or cheating customers out of their money, then it's not wrong to do so. What's right is what's good for me. This is the reef upon which any any ethics based on the assumption of naturalism will inevitably founder.
Naturalism leads to egoism and egoism leads to "might-makes-right". It's a very bleak basis for human flourishing, but that's what you get in a world that no longer believes in God.