The moral theory that is based on the claim that morality comes from religion is called Religious Authoritarianism or Divine Command Theory. It means that religion is the authority on moral matters. As you might already suspect, religious authoritarianism has some of the exact same problems as cultural relativism. The problem that is most similar to cultural relativism is that not everyone shares the same religion, so different people would come up with different answers to the same moral questions. And if ethics is based on what all people have in common, religion isn’t going to work. Furthermore, there are lots of people who have no religion at all. If we say that each person answers moral questions my referring to her religion, what are the non-religious supposed to do?
The obvious answer that religious authoritarians will give is that their religion is the correct one, and that everyone else who makes moral decisions based on another religion (or no religion) is mistaken. This brings up the problems that are unique to religious authoritarianism. The first problem that is particular to religious authoritarianism is that religions come with a lot of supernatural claims built in. For someone to accept that religion is the source of morality, they would first have to accept that the supernatural claims of that religion, because the moral commands of any religion are usually issued by some supernatural agent. Most religious authoritarians believe in the existence of some deity that gives commands, and that those commands are the basis for moral decisions. So there is no morality without that deity. The problem is that if a religious authoritarian was asked to defend his moral views, he would not only have to explain that there are commands issued by a deity, he would have to convince his audience to accept the supernatural claim that his deity exists in the first place. That’s a tall order if his audience is someone who is not a believer in that religion already.
A further problem is that even people who believe in the same deities might still not agree about what those deities command. Frequently this is because understanding the commands of deities for most religions are a matter of interpreting a holy text, and interpretations differ. If a holy text describes a deity’s command such as “do not kill”, does that mean do not kill innocents, or do not kill intentionally, or do not kill anything under any circumstances? Does it include seemingly insignificant things like bacteria? It’s not clear how particular commands like that should be interpreted. Further, what should we do in cases where religion provides no explicit commands? Most religions have no holy texts that address the moral permissibility of texting while driving, or software piracy.
Even if we set aside the problems of possible misinterpretation, supernatural claims, and disagreement between religions, there is still a serious conceptual problem with using religion as the source of morality. It’s a problem with the way religious authoritarians understand what moral rightness and wrongness mean.
Plato identified this problem in his dialog Euthyphro, in which Socrates confronts a scholar named Euthyphro, who claims to be an expert on religion. Socrates asks Euthyphro what piety (good religious behavior) is like, and Euthyphro responds that pious behavior is behavior that is loved by God. It this passage, Socrates wonders whether what is holy (pious) is holy because it is loved by God, or if it is loved by God because it is holy.
SOCRATES: And what do you say of piety, Euthyphro: is not piety, according to your definition, loved by all the gods?
SOCRATES: Because it is pious or holy, or for some other reason?
EUTHYPHRO: No, that is the reason.
SOCRATES: It is loved because it is holy, not holy because it is loved?
SOCRATES: And that which is dear to the gods is loved by them, and is in a state to be loved of them because it is loved of them?
SOCRATES: Then that which is dear to the gods, Euthyphro, is not holy, nor is that which is holy loved of God, as you affirm; but they are two different things.
EUTHYPHRO: How do you mean, Socrates?
SOCRATES: I mean to say that the holy has been acknowledged by us to be loved of God because it is holy, not to be holy because it is loved.
SOCRATES: But that which is dear to the gods is dear to them because it is loved by them, not loved by them because it is dear to them.
SOCRATES: But, friend Euthyphro, if that which is holy is the same with that which is dear to God, and is loved because it is holy, then that which is dear to God would have been loved as being dear to God; but if that which is dear to God is dear to him because loved by him, then that which is holy would have been holy because loved by him. But now you see that the reverse is the case, and that they are quite different from one another. For one [theophiles] is of a kind to be loved cause it is loved, and the other [osion] is loved because it is of a kind to be loved. …
I’ll put Socrates’ question another way, to make it simpler. “If some particular action is good, is it good because God commands it, or does God command it because it is good?” This dilemma illustrates the central conceptual problem with religious authoritarianism. Let’s think about what each of the two options presented in Euthyphro’s dilemma would mean. I’m going to start with the second half first.
If God commands some particular actions because those actions are good, then they must already have been good before God commanded them. That is, God is simply identifying the actions that are already good and commanding that we do them. Perhaps God is a very reliable detector of goodness in actions, but the goodness does not come from God, so religion is not the source of morality.
If actions are good because God commands them, then any actions God commands would be good. God could command murder and theft, and those things would become good because God commands them. If whatever God commands is good, then God’s commands become arbitrary. They are not based on any reasons; they don’t have to be, because God doesn’t need reasons. God could just make up reasons but none of them matter because on this view God can’t be wrong—whatever God commands is good because God commanded it. God could command evil things and they would become good, and there is no reason to think that God wouldn’t command evil things, because God’s commands aren’t based on anything other than that fact that God commanded them.
Neither of those options are particularly appealing, especially to people who should be sympathetic to religious authoritarianism, because on the one hand, God is unnecessary, and on the other hand, God is arbitrary. Most religious believers will say that God’s commands are not arbitrary, and God would not command evil acts. They would claim that God makes commands based on some principles. If that’s true, then we can still ask what those principles are, and whether they actually come from one of the other sources of morality.
 Plato, Euthypro, trans. by Benjamin Jowett, Project Gutenberg collection. http://www.gutenberg.org/files/1642/1642-h/1642-h.htm.