I'd challenge whether moral realism can be true even if physicalism is false. Even if a god says you should do something, you still just have an "is" statement, i.e. "God says X", from which you can't get a "should", i.e. "You should do X". The way theists try to avoid the problem is by just defining God as good, but that renders it kind of meaningless to say such things are "moral" or "good", because that can include literally anything. A physicalist moral realist could be potential do something similar by e.g. defining suffering as bad and pleasure or the survival of the human race as good.

@NotThatGuy because that can include literally anything - This statement appears to assume that on theism morality would be a brute contingency (i.e. "arbitrary"). But I don't think this is how theists usually view morality. On theism, what is good or moral is usually seen as metaphysically necessary, not contingent. (There is debate though, see onlinelibrary.wiley.com/doi/full/10.1002/tht3.494) From the viewpoint of a physicalist moral realist, how would morality objectively exist in the physical realm?

What I mean is that if you define some being (and, by extension, everything they command) as necessarily good, then that being can command you to throw babies off a building for no reason at all, and you'd have to say that is good merely by virtue of them saying it, which makes the action itself arbitrary: it doesn't matter what effect it has on anyone, it just matters who said it. The above might sound like an extreme example, but a whole lot of people (including children) have been murdered because people thought God told them to do that. Whether God would say that is a subjective evaluation

This may just be my opinion, but some people egregiously abuse terms like "necessary" and "possible" to try to argue in favour of their desired conclusion. If someone can't concretely define what that means as an actual feature of reality, where and how it exists, why it's necessary or possible, what it means to be necessary, etc., then I'm inclined to just dismiss them as speaking gibberish. No matter how much deeply I investigate their claims, it all being gibberish is generally the only conclusion I come to.

@NotThatGuy You seem to be critiquing the arbitrariness of divine command theory as one of the two alternatives posited by the Euthyphro dilemma. However, there are responses to this critique, for example, by arguing that this is a false dilemma. "Anselm, like Augustine before him and Aquinas later, rejects both horns of the Euthyphro dilemma. God neither conforms to nor invents the moral order. Rather His very nature is the standard for value."

@ Mark It's wrapped up in smart-sounding words, but that seems to just be asserting God is good by definition, which doesn't seem to address my arbitrariness objection at all. It's just philosophical ramblings with no practical implication. If God tells you to murder a bunch of people for no reason, does that mean that's good and you should? If God wouldn't command that, because it's not good, then you'd have to argue what "good" is independent of God, at which point you've made a subjective case for morality, and attributing that to God just makes it sound more objective than it actually is.

@NotThatGuy If God tells you to murder a bunch of people for no reason, does that mean that's good and you should? - According to Divine Motivation Theory (DMT), he wouldn't, because it would go against God's motives. The only exception would be in the case of decreeing judgement, but that would be a reason, not lack thereof. If God wouldn't command that, because it's not good, then you'd have to argue what "good" is independent of God - Why independent of God? I'm not following the logic here.

I see no reason for it to be "independent". As I said earlier, you seem to be appealing to the Euthyphro dilemma, but that's a false dichotomy. For instance, Divine Motivation Theory would say that God wouldn't command it because it is not good, because it goes against God's motives (i.e. His nature). It's not independent of God, but entirely based on His nature.

at which point you've made a subjective case for morality, and attributing that to God just makes it sound more objective than it actually is - I'm not really sure what you mean by "subjective case for morality". But a Christian theist would probably say that God has revealed (part of) his views on morality through Sacred Scripture, and also through the ongoing influence and guidance of the Holy Spirit (depending on the degree of consecration of the Christian and the intimacy of the relationship they have with God), thus providing an objective anchor for morality.

it would go against God's motives But that's impossible to know because "God works in mysterious ways". If you don't accept that God's motives are unknowable, then you have to defend against other criticisms. Also, suppose someone who claims to be God (e.g. the voice in the head) commands you to do it. How would you know it's not good and therefore does not come from God?

God has revealed (part of) his views on morality through Sacred Scripture Not really, because he was asking prophets to do something that contradicted the commandments: i.e. his present word takes precedence over the commandments. And some of these tasks would be seen questionable by many. As for epistemology, it does not seem that Reformed Epistemology tells you how to separate God from the devil (who tries to convince you he is God).

@rus9384 Not really, because he was asking prophets to do something that contradicted the commandments: i.e. his present word takes precedence over the commandments. - Examples please? As for epistemology, it does not seem that Reformed Epistemology tells you how to separate God from the devil (who tries to convince you he is God). - This is probably relevant: en.wikipedia.org/wiki/Discernment_of_spirits

@rus9384 Now, if you mean being deceived by a Cartesian evil demon, in that case we are essentially screwed, and that would lead to solipsim.

@Mark Ok, it doesn't have to be independent of God, but a dependent argument seems much, much weaker. An "evil" Christian could also say God revealed his character through scripture, e.g. all the times he commands the slaughter of other people. And they could just say the holy spirit tells them different things than what you say it says. Basing your morality on your interpretation of a book written by flawed humans has too many problems. Also, what about non-believers?

There are plenty of people who are physicalists and also moral realists, intelligent people even. Whether they're correct or not is... well, obviously debatable. I personally think that, IF moral realism is a tenable position at all, it must be compatible with physicalism, but I'm not convinced moral realism is true or even tenable yet.

@Mark The Binding of Isaac precedes the commandments, but you'd think God's morality is permanent, not subject to change over time. Discernment of spirits is not useful when the devil says what you'd expect God to say. E.g. he asks you for a sacrifice, to sell your possessions, etc. All of those can lead to favoring the devil, even if it's not obvious to a human.

@Mark It's interesting that you'd accept an deceptive demon as a problem. What about a deceptive mind? What if the deception is not deceiving you about all you perceive, but instead you have some cognitive biases that's making you interpret some things as evidence of God's existence or signs from God when they actually aren't? In this case, one could still be justified to generally trust perception, and it also beautifully explains differing beliefs across the population and invalidates theological problems.

@NotThatGuy And they could just say the holy spirit tells them different things than what you say it says - This circles back to rus9384's objection that how can we tell that we are not being deceived by the devil (or our minds, to entertain your suggestion as well). If a Cartesian evil demon exists, we are screwed. The same goes for brains in a vat, the Matrix, etc. Now, if God exists and only grants limited freedom to the devil, then theodicies apply, reformed epistemology might apply, discernment of spirits might apply, consecration and intimacy with the Holy Spirit might apply, etc.

@rus9384 Regarding the binding of Isaac, there are some theodicies here: christianity.stackexchange.com/q/84176/61679. Regarding being deceived by the devil, see my comment above in response to NotThatGuy

@Mark Yes, I am aware of the theodicy, however the devil can "test your faith" in similar ways. It does not need to be a Cartesian evil demon. In fact, you can't even tell which of the religions and denominations is more truthful (if there is any truthful one), because quite literally, if there are mutually contradictory monotheistic religions (based on some lore), at least one of them has to be false, i.e. the god of that religion must be fake. But you can't tell which one.

@rus9384 But you can't tell which one - justification needed.

@Mark I mean, there are people out there doing horrible things because they think God told them to. And entire societies did that throughout history. So this isn't really a hypothetical as much as it's a reality. If one truly wishes to know God's character and wishes, rather than just following one's own desires, it seems necessary to apply extreme skepticism to anything one thinks God told them, how they interpret the Bible and how much weight they put on that. But I imagine one would have a really hard time resolving that skepticism (other than to conclude you can't know anything about God).

@Mark If you can, you just need to provide evidence for that. But yes, I can weaken my claim to "No one has yet provided a convincing argument for the specific religion."

@rus9384 But yes, I can weaken my claim to "No one has yet provided a convincing argument for the specific religion." - Fair enough. Thanks for the weakening. But in this case, judging whether certain arguments or pieces of evidence are convincing or not becomes rather subjective, so I would further qualify your claim with "for me". In the end, it all boils down to one's epistemology. In Christianity, there are different epistemological approaches to how one can be convinced. I list several alternatives in this answer.

@rus9384 This question might be of interest as well: christianity.stackexchange.com/q/99924/61679

@Mark Though, these only are arguments for existence of God, not for Christianity being truthful (e.g. it could be God exists, but the Church has gone astray and the texts were spoiled).

@rus9384 The latter link is more specific (well, technically the former also references different versions of apologetics, which are more specific to Christianity as well)