Meta-ethically, divine command theory is arguably a species of ethical subjectivism rather than moral realism because it claims that moral norms and values are derived from the subjective preferences of one specific intelligent being, the purported creator of the universe. However, for humans who, according to theists, are commanded to follow the moral norms dictated by the purported creator, those norms would appear as facts of the universe not reducible to natural properties (though arguably reducible to purported supernatural properties such as divine speech). Thus, from a human perspective, divine command theory is indistinguishable from moral realism. It is only from the divine perspective, supposing the being who would have this perspective actually exists, that divine command theory is a variety of ethical subjectivism.
Even though many people believe such a being exists, there is no evidence for an intelligent creator god who created a set of norms people ought to follow, sustains and interacts with the universe, and sustains the norms people ought to follow. Even if it is possible that an intelligent being created or gave rise to the universe, there is certainly no evidence that this being ever revealed itself or its will to humans. All stories about a god revealing itself or its will to humans are unreliable because they contain elements that depart so much from ordinary human experience that they cannot be trusted, there are countless versions of such stories which contradict each other on key points and there is no known consistent and reliable method for evaluating the truthfulness of their non-historical claims, and most importantly, none of them have been corroborated by any observable evidence in the present.
The above argument disposes of divine command theory, as there is no way to posit this theory in the absence of evidence for a creator.
However, theists frequently make a sort of utilitarian argument to defend divine command theory. They hold that without a creator to set up and sustain moral principles, (1) moral principles are not binding but optional, and (2) if moral principles are optional people will feel they have license to violate them whenever they want, which will lead to mass immorality. This argument is typically used as an attempt to convince nonbelievers to believe in the existence of a creator (usually a particular version of a creator), but it is also used in moral discourse as an argument against both moral relativism and nontheistic varieties of moral realism.
Here, I will focus on the way this argument is used in moral discourse. When used against nontheistic varieties of moral realism, the argument is really that it is easier for a theist to justify moral realism than it is for a non-theist to do so. It is an argument about convenience, not about facts, for in such a situation the theist and the nonbeliever already share a belief in moral realism and only disagree about what facts would justify moral realism. In such a situation, the theist does not argue for moral realism, or even for the existence of God, but rather that nontheistic varieties of moral realism are not really moral realism at all because they don’t provide a convincing metaphysical justification for moral realism.
On this point, I actually do agree with the theists that nontheistic varieties of moral realism do not provide convincing grounds for believing in moral realism. However, divine command theory doesn’t fare any better than the nontheistic varieties of moral realism. While it provides a justification for believing in moral realism, unless that justification (the existence of an intelligent creator who interacts with the universe) is demonstrated to be true, then it does not as a matter of fact provide any grounds for believing in moral realism, even if it could provide such grounds in principle.
On top of that, the only way the norms set up by the hypothetical creator would be “binding” is if there were consequences for failure to abide by those norms. Of course, many varieties of theism teach that the consequences for such a failure are of the most severe sort imaginable, consisting in eternal or at least very long-lasting torment or agony. However, if one acted with a purpose of avoiding such consequences, one’s actions would be out of self-interest and ultimately would have nothing to do with moral realism. The logic at work in such a situation would (or at any rate could) be more like the following: “a very powerful being who has complete control over my entire being has ordered me to behave a certain way and threatened to torture me if I don’t; I will do it to avoid torture, whether I think I ought to behave that way or not.” Of course, it would also be possible to follow that powerful being’s commandments out of love (albeit of a slavish, blindly obedient sort) or a sense of duty, rather than out of fear, in which case theistic morality would be compatible with moral realism. But again, there is no basis for believing in any theistic morality without evidence for the existence of an intelligent creator and evidence that the intelligent creator one believes in commanded the specific moral norms one believes in.
Some atheists and secularists would probably argue that this reasoning plays right into the hands of theists. But at best it would only establish that it would be more convenient if some form of theism were true, though I don’t think it would even establish that in most cases, as there are many drawbacks to almost every form of theism, particularly those which teach damnation of sinners, which would arguably outweigh any possible moralistic benefits.
Of course, deism, which I understand to be belief in an intelligent creator who set the universe in motion but does not intervene in it, is also a possible basis for affirming moral realism. However, deism as I understand it is utterly inconsequential to our daily lives and is compatible with a secular lifestyle. In fact, because deism rejects the unprovable elements of religion and categorically denies the existence of miracles, it is in many ways even more incompatible with any form of theism than it is with atheism or secularism.
The dilemma cited in Plato’s Euthyphro, namely whether the supremely powerful and intelligent being (supposing such a being exists) does what is good because it is good by some pre-existing standard, or whether what that being does is good because that being does it, has caused great controversy among philosophical theologians. However, this dilemma does not arise in the so-called “sacred texts” of the major theistic religions, such as the Bible and Qur’an, and it need not arise in philosophical debate.
Theism is the view that a supremely powerful and intelligent being is the creator of all things, including the moral principles people are to live by. Theism thus unambiguously entails divine command theory. For if morality existed prior to the supremely powerful and intelligent being posited by theism, this would mean that something greater than that being and binding on that being pre-existed that being, which would negate the most basic teaching of theism, that a supremely powerful and intelligent being is the creator of the universe.
While I certainly do not consider such “sacred texts” authoritative, on this point they are correct. If morality existed before the posited supremely powerful and intelligent being, that is, if that being did what was good because it was good by some pre-existing standard, this would be the negation of theism. The reasonable thing to do in that situation would be for humans to follow the pre-existing moral law and ignore any being holding itself out as the supremely powerful and intelligent being.
Of course, it would be possible to posit that the purported supremely powerful and intelligent being created everything but morality, and further that that being always acts consistently with what is good. But metaphysically this would still be an unambiguous rejection of the basic premises of theism, and meta-ethically it would entail a moral realist theory other than divine command theory (likely non-naturalism but possibly naturalism).