The likely answer from a religious person as to why we should not steal, or commit adultery is: “because God forbids us”; or if we ask why we should love our neighbour or give money to charity then the answer is likely to be “because God commands it”. Drawing this link between what is right and wrong and what God commands and forbids is what is called the Divine Command Theory (DCT).
There is a powerful and influential challenge to such an account called the Euthyphro dilemma after the challenge was first raised in Plato’s Euthyphro. The dilemma runs as follows:
Either God commands something is right because it is, or it is right because God commands it. If God commands something because it is right, then God’s commands do not make it right, His commands only tell us what is right. This means God simply drops out of the picture in terms of explaining why something is right.
If on the other hand something is right because God commands it then anything at all could be right; killing children or setting fire to churches could be morally acceptable. But if a moral theory says this then that looks as if the theory is wrong.
Most theists reject the first option and opt for this second option — that God’s commands make something right. But they then have to face the problem that it make morality haphazard. This “arbitrariness problem” as it is sometimes called, is the reason that many, including Aquinas, give up on the Divine Command Theory.
So for Aquinas what role, if any at all, does God have when it comes to morality? For him, God’s commands are there to help us to come to see what, as a matter of fact, is right and wrong rather than determine what is right and wrong. That is, Aquinas opts for the first option in the Euthyphro dilemma as stated above. But then this raises the obvious question: if it is not God’s commands that make something right and wrong, then what does? Does not God just fall out of the picture? This is where his Natural Law Theory comes in.