Let's assume that a God does exist
That's quite a thing to assume, and for us to base our morality on that, we may also need to assume that we have sufficient justification to rationally conclude this (and I don't consider us to have that).
Different motivations behind morality under theism may depend on God having certain traits, and the existence of souls or an afterlife (and I'd consider this to not just lack evidence, but to largely be contrary to the evidence).
And there's also a significant problem of knowing what God wants. Even if a god exists, it's undeniably the case in the world we're living in, that there are vastly different opinions about what God commands (even within the same religion, especially across time).
That seems like a necessary caveat to start with. But on to the actual answer.
It's just assertions
As far as I see, people only assert that something is objective morality, but no-one has ever been able to (rationally) conclude that something is objective morality.
People assert:
- That "God's very nature is itself the objective standard of goodness"
- That the morality of obeying one's creator is objective morality
- That the morality of obeying God to avoid eternal torture or gain eternal bliss is objective morality
- That the morality which God has "written onto our hearts" is objective morality
- (There are also assertions about some natural things being objective morality outside of theism)
But these are just assertions. Subjectively, someone thinks that the above things are objective morality, but I don't recall (at least not at this exact moment) having ever seen anyone actually even try to explain why that's objective morality. It's just asserted.
It's not at all clear why a creator should have absolute (or any) moral authority over their creation. Parents don't have absolute moral authority over the children they "created". If a human creates a conscious robot and commands it to indiscriminately slaughter countless innocent people, it's possible that the robot's programming may prevent it from refusing (in which case it's not morality because it's not a choice), but if it can choose, I would not say that the robot must listen to its creator merely because it was created by them (even if the creator is able to erase the robot from existence or if they would reward them for obeying).
Eternal consequences is one is the only one that gets close to actually giving us something approach a good reason to follow that morality (even if that's a selfish reason). But that's still very much subjective (even if it might get close to being universal, if we can all agree that some god exists and what they command, which we definitely don't agree on).
The is-ought problem
Anyway, let's take a step back and ask: what is morality?
I would define morality to roughly be what we "should" or "shouldn't" do, specifically in relation to how we treat others, subject to some principle or goal. And this is commonly put forward as the recommendation for how everyone in a society should behave.
If you say we "should" do something, without some principle or goal, you run into Hume's is-ought problem of being unable to get a "should" from an "is".
Putting aside the fact that our perception of reality is subjective, if we're talking about objective reality, we can list of facts all day, but you're unable to cross the threshold from factual "is" statements to a moral statement about what you "should" do.
The only way it seems possible to get to a "should" is to add a goal:
- I am hungry. ("is" statement)
- I desire to no longer be hungry. ("is" statement)
- Eating a sandwich will stop me from being hungry. ("is" statement)
- I should eat a sandwich >IF< I want to satisfy my desire to stop being hungry. ("should" statement, with a goal)
The final statement with the "should" wouldn't follow from the statements above without the "if...". It might be the case that I'd rather eat something else, or that I don't have any bread, or that I have competing desires such that satisfying my hunger is not my immediate priority.
For morality, we also need to add a goal to get to a "should". Goals are subjective and dependent on the moral agent.
To say that something can be moral objectively, independent of a moral agent and independent of a goal - that seems incoherent. At that point it's not clear what we're even talking about any more.
God or no god, you can't get around that.
* If one says that God is the moral agent or chooses the goal or imposes it upon us, then that's still just subjective morality (whether someone asserts that to be objective or not). It doesn't solve the is-ought problem.
Do you really have a "should" when you have a goal?
Let's consider the statement above:
I should eat a sandwich if I want to satisfy my desire to stop being hungry.
We could rephrase this as:
Eating a sandwich will satisfy my desire to stop being hungry.
This communicates roughly the same sentiment, but the key difference is that the latter is an "is" statement.
We are making a factual statement that an action will result in some consequence, and the "should" becomes implicit based on whether that's a desirable consequence (all things considered). One could say that the "is" statement is no longer prescriptive and telling someone what to do, but the "should" statement was never truly that either - the "should" was entirely dependent on the consequence being desirable.
Any goal-based "should" statement can similarly be rephrased as an "is" statement.
This seems to leave the idea of an objective "should" in even more dubious territory, because I don't see any way to similarly convert such a statement to an "is" statement. The objective "should" is left as something that's incomparable to anything else, which has no frame of reference for what it means, in which case it's not far about gibberish.
Objective morality, attempt 2?
As a bad example of a moral standard, let's say whatever action results in you getting the most grains of rice is moral.
Now we could say that we can objectively measure the number of grains of rice that some action produces. So it could be said to be "objective" in that sense. But this doesn't solve the is-ought problem. We can't say we objectively "should" do something just because we can objectively determine that that thing will give you the most grains of rice. We need to first agree on the moral standard of trying to get the most grains of rice, and that is the is-ought problem and the problem with objective morality (which this doesn't solve).
To relate that back to the question, some people say we can objectively determine whether something matches what God wants (to which my immediately response is: no... how? ... what? Have you seen how many people disagree about what God wants...). But even in the best case, this still runs into the problem described above, of giving you an measuring stick that can be objectively measured, but not an objective justification for using said measuring stick.