Theists often claim that morality can only come from God. It’s a claim that I’m pretty tired of hearing and is also pretty irrelevant for any sort of debate on the existence of God (despite whatever certain people might try to tell you).
In their most sophisticated version, what theists mean is that morality can only be metaphysically grounded in God and not that only religious people can be moral. Nonetheless, most philosophical views about the metaphysical grounding of ethics are secular. For some examples, give these a quick google search:
1. Ethical non-naturalism (for example, David Enoch’s view in “Taking Morality Seriously”)
2. Contractarianism (for example, John Rawls’ view)
3. Utilitarianism (John Stuart Mill)
4. Neo-Kantian Deontology
5. Naturalistic moral realism (for instance, Richard Boyd’s view in “How to Be a Moral Realist”)
6. Humean Conventionalism
7. Non-theistic Virtue Ethics (God isn’t necessary, for example, in Aristotle’s “Nichomachean Ethics”)
And there are several others as well.
Secular people don’t need to choose any of (1)-(7), and it’s worth noting that these options are available to Christians as well (some Catholics are virtue ethicists, for example). They don’t include God in their descriptions of morality, but one could incorporate them into a theistic world view nonetheless. However, the fact that these and other views *exist* means that it isn’t problematic for a secular person to think that morality exists and that it’s non-relative. All they have to say is, “Look, I don’t know which moral theory is correct. But here’s a big list, and I’m pretty sure that something like one of these is actually the case, even if none of these is precisely the true story. Besides, we have good reasons to think that moral relativism and moral nihilism cannot be the case.”
The truth of the matter is, there are a lot of mysteries in our world. Both theists and atheists actually agree on this point; theists can point to the mysterious ways of God, for example, while atheists can point out that there are scientific and philosophical mysteries that we have yet to solve. Usually, theologians think that a god-of-the-gaps is not a very good god; the gaps in our knowledge are always shrinking, so a god put into those gaps is made ever smaller. So the fact that we haven’t yet figured out everything that there is to know about morality shouldn’t be a reason to think that God is responsible for it; that would just be another god-of-the-gaps. Of course, theists can posit God as a potential explanation of morality. But it’s just disingenuous to claim that that’s the only possible explanation.