Paraphrased for brevity and clarity:

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Atheist: But you are assuming that there has to be some objective moral “truth”.  That’s just an opinion.  Morality is just about bringing the greatest good to the most people.

Me: Here is a fact: If your last statement were true, then it would be an “objective moral truth”.  Here is another fact: without a higher-than-human moral authority, any human “morality” is simply that person acting according to their own nature.  If “morality” is defined as “acting according to one’s own nature”, then “morality” becomes a meaningless term.  Since no human agency has greater moral authority than any other, the only way that morality has meaning is with the acceptance of the existence of God.  An atheist can certainly be ETHICAL, but he cannot be MORAL.

Atheist: You don’t need God to be moral.  It’s just about not increasing suffering.

Me: But without a higher moral authority, all morality—including that statement–become mere opinions.  I know of several groups which would disagree with your opinion VIOLENTLY; and without a higher moral authority, their opinion is every bit as valid as yours.

Atheist: Are you suggesting that ISIS has a seat at the table of a discussion on morality?

Me: Yes; if there is no higher moral authority than a human, then every human has an equal “seat at the table on a discussion of morality.”

On that topic, let’s discuss your fascination with 18th century utilitarianism.  You have made two statements regarding utilitarianism–let’s see how morally coherent they are.  We’ll start with the second: “it’s about not increasing suffering.”

Scenario: You come across a rape in progress.  Do you intervene?  Either way,  the victim has already been raped–but if you act to defend the victim, you risk injury both to yourself and to the rapist, either of which would increase suffering.  So, by your philosophy, the rapist should be allowed to finish–and in fact not prosecuted, since both prosecution and incarceration would cause him to suffer.

Or, how about murder?  Suppose you come across an indigent with no one to miss him.  By your philosophy, there is no moral argument against murdering him–he will no longer suffer anything after he dies, and no one will miss him.

Note that this also applies to any group of people large enough to close the circle of anyone close enough to grieve over a death; as long ENOUGH PEOPLE are killed, your philosophy has no moral objection.

But what about your first statement: “bringing the greatest good to the most people”?

Well, we still have trouble with stopping a rape-in-progress.  While we are certainly bringing good to the victim, we are also still posing the possibility of harm and even death upon ourselves and the rapist by trying to stop the crime.  Moreover, the sort of person who commits rape derives from it not only the physical satisfaction of sex, but the mental satisfaction of domination–both of which we would be denying them by interrupting the rape.  Our moral paralysis would probably result in the completion of the crime, regardless.

But an interesting thing happens when we look at the rape itself: we can say that pleasure of the rapist (as a good for that person), and the suffering of the victim (as a non-good for that person), roughly balance the scales.  Or, we may even put the balance on the side of the victim.  We may even do so by a significant margin.  But what if there are more than one rapist?  Now, many people can receive “gratification” from the same act, while only one person suffers.  Your philosophy not only condones this action, it CALLS for it.  And the more rapists, the more ethical this action is, in the view of utilitarianism.  It would also be more ethical to murder the victim after the fact, so that they no longer suffer…

What of murder in general?  Entirely permissible under utilitarianism, so long as the person was more disliked than liked.  But how about ETHNIC CLEANSING?  Sure, the minorities will object–until you’re done killing them–but their objections are irrelevant to the utilitarian.  The majority is, by definition, the greater number of people–so as long as THEY FIND THE ACTION BENEFICIAL IN ANY WAY, it is a moral action according to utilitarianism.

As much as progressivist-atheists like to whine about “internalized racism”, “internalized misogyny”, and all of your other made-up nonsense, your idea of an ethical system only works if everyone in the world behaves according the pre-existing code of conduct to which you are accustomed from growing up in a Christian society.  What you have just demonstrated is straight-up INTERNALIZED CHRISTIANITY.  Congratulations.