I find that the best demonstration for this, ironically, is what may well be the most common response materialists give to the moral argument: the Euthyphro Dilemma.
The Euthyphro Dilemma is, in a nutshell, the question “Is something morally good because God commands it, or does God command it because it is good?”. If the former, then it seems that morality is arbitrary; if the latter, then it seems that the real standard of goodness is something other than God.
Now, it’s long since been pointed out that this is a false dilemma. It leaves out the significant option that God’s own nature is the source of goodness (which is what monotheists have traditionally claimed). That is, God commands what he does because it is right, but the “source” of rightness is his own nature. There’s no need to assume that such a thing must be separate from God. Thus, the dilemma is answered. What is the source of goodness? The nature of God.
But I think the theist can do better than this.
For, while this answer works quite well for monotheism, it is a very difficult challenge for every other system of ethics I know. Utilitarianism, the Veil of Ignorance, Natural Law, etc. have been unable to answer Euthyphro-like questions without appeals to either God or Platonism (which is well outside of materialism).
For instance, the Utilitarians claim that we should do what brings the greatest happiness for the greatest number of people. Most of us would see that as a worthy goal (though there is trouble in the details). But, one could easily ask “Should we seek the happiness of others because it is good, or is something good because it brings happiness?”. There doesn’t seem to be a way to argue that human happiness is the standard of moral goodness in the way that a transcendent God’s nature is.
Hence, moral good becomes relativistic under utilitarianism.
And this is why the majority of materialists are moral relativists. The Euthyphro is much easier to answer if there is some absolute, objective goodness out there (like God). But, even if one believes in moral truth, explaining morality seems to require theism.
So, it strikes me as odd that the Euthyphro is most often heard from proponents of atheism. In my mind, it is one of the best reasons to believe in God.