I am a firm believer in God and believe that morality is certainly derived from Him and Him alone...
I'm an atheist, and I don't understand how a believer can submit/admit to something like the above.
Sorry if you consider my analogy to be condescending, it isn't meant to be but I can understand that someone might read that into it.
Here goes.
I consider the position that "morality comes from god" to be akin to Adults vs Children.
A child might consider that something is "wrong" because either Mum or Dad or the Teacher or other TELLS them that it is "wrong". The child is put into a position of submission, of being deemed not capable of understanding why something might be "wrong" or even not being capable of judging something as "wrong" unless they are told by this authority. And to some degree, under law we give children much leniency and don't convict them of crimes as we would an adult.
But as an Adult, we expect people to have sound judgement and to behave responsibly because we deem that they ought to have the faculties to determine if some action is "wrong" or not and to hence make informed and calculated decisions.
Now, I don't for one second believe that a Christian thinks it is wrong to commit murder simply because they believe their god tells them that this is wrong. I expect that Christians are quite capable of coming to a conclusion that "murder" is "wrong" and its not something that they would question with their god if they had an opportunity to have an open, honest and full inquiry with god.
But me trying to square these two things "1. a Christian believes that morality comes from god", "2. a Christian adult doesn't need god to tell them that murder is wrong, in order to accept that murder is wrong". I can't square them, item 1 tells me the Christian doesn't think, but instead submits to the total authority of their god. But item 2 tells me that Christians do think for themselves. In reality, I think Christians think for themselves.
that being said, however, I'm wondering how a person would debate this with someone like an Atheist?
It depends what you are trying to debate.
1. Are you debating that morality is objective but also comes from God?
Most atheists would probably argue that if morality comes from god then it is subjective to god. Unless of course, god is a predictable rules engine rather than an unpredictable consciousness.
2. Are you debating that morality is objective?
Most atheists would probably argue that there are many grey areas of morality that people disagree on, many situations where people need to make difficult choices etc.
3. Are you debating that morality is objectively discoverable?
As an atheist I would be keen to learn how it is discoverable
4. Are you arguing that a person can't have a moral standard or belief without a belief in god?
Isn't it conceivable that mature adults can come up with rules that help a society of people to live together in relative harmony?
since the truth is objective and not just some kind of malleable or subjective reality.
If both truth and morality are objective then they can't come from god.
how would someone discuss this point with an Atheist who clearly does not believe in God and seems highly unlikely to cave in to the idea?
Clearly define what you are debating, be respectful, listen and ask for clarification rather than assume the other person is wrong.