Which doesn't mean they aren't moral, so your attempt to explain them away to engineer your desired conclusion is laughably wrong. Sorry they weren't carved in tablets of stone and delivered from on high, but not all moral systems work that way. That doesn't mean they are incoherent just because they have a use other than stroking a deity's ego.
There is a difference between establishing a system based on politics (that is, to ensure coexistence between members of a society) and a system based on moral value.
The first is truly rational because it maximizes benefit for all members of society, and yet it is confined to the realms in which society actually plays a role or has an enforcer. That is to say, should you ever attain a position of power or independence where society has no means (or knowledge of the crime) to effectively sanction you, it would be very rational to act only on own benefit, ignoring the rules/traditions/laws.
The latter is wholly irrational because it presupposes that some actions are inherently good or inherently wrong. Whether one comes to that conclusion because one believes in a deity or not is irrelevant - both conclusions are irrational.
It's blind assertion. Come lecture me when you have something other than that.
Your arguments are based on wishful thinking. You suppose Christianity (or any other belief in a higher deity) is irrational, yet are unable to argue why you believe that it's wrong to rape a woman or rob a millionaire if you'd get away with it.
Atheists who claim to only believe in things for which there is solid evidence should have a fundamental knowledge in how science works. That is to say they should either educate themselves on the principles or (better yet) partake in research themselves.
Again, I don't want to be insulting, but from your argumentation (and lack of formal scientific understanding), I have a gut feeling that you have done neither.
Why are we talking about "existence"? We are evaluating the morality of actions. They certainly exist as ideas, but not in any physical sense.
The actions exist in a physical sense. The question is whether there is any evidence to believe the actions themselves have properties that would qualify as "good" or "bad".
2.1 - And you've simply dismissed things that would serve individual benefit that don't suit your argument - like a sense of empathy.
I am not saying it's
always irrational not to rob a millionaire. It's
sometimes irrational - and yet most Atheists will still not rob a millionaire even if it were rational.
You're trying to discredit an argument by arguing the cases that have been formally excluded. It would be like you saying: "Abortion isn't good if it's done when the baby is 38 weeks old, but before that it's fine" - and I answered "But you've ignored that some babies are 40 weeks old! It's totally unethical to kill a baby that's 40 weeks old!"
2.2 - Might? Well, good to see your argument is solid.
As I said, I am not saying that Atheists are universally irrational.
There are some instances in which a decision not to rob a millionaire are rational (he's got guns, security, the law on his side). And there are some instances in which it is not. And yet, most Atheists will argue that even in these instances, robbing the millionaire is wrong.
Except these actions aren't just emotion.
Also - again - unlike your deity - emotions exist
And you're managing to be condescending and preachy. Bang-up job.
All you're doing is repeating the mantra of "that doesn't apply to
all situations" or "there are other reasons! I just don't want to tell them to you!".
With all due respect, all I'm doing is showing that it's as irrational to believe in a higher deity as it is to believe in atheist morality.
I'm saying we're equals. You have your beliefs, I have mine.

The fact that you are clinging on to your supposed feeling of intellectual superiority is fascinating and sadly goes to show that some Atheists are rooted not in simply "not believing in God", but "believing in their own superiority". This might be the reason for some of the backlash the OP may be getting.