"My real objection is that one of the premises of Christianity is that all people are sinners. Define sinner any way you like, it's not a premise I agree with."
But it isn't a premise, at least not strictly speaking.
To you use your analogy, the Christian is told he can't fly and accepts it. The atheist investigates this so-called "reality" and invents the airplane
The only problem with this is that the rest of your post undermines this. I am keeping in mind that you were at one time an Objectivist and presuming you still are, so correct me if I'm wrong about that. But going on that assumption, a comment like this undermines your argument:
I never said I always make the right choice, just that there is always a choice to make.
Christians say exactly the same thing. Why don't you
always make the right choice? And why should you feel bad or out of joint if you didn't? And don't tell me you don't care otherwise you wouldn't have included the caveat that you never make the right choice. You clearly wanted to preserve yourself from the accusation that you think yourself perfect. Are you perfect, Ragnar? If not, what is the opposite? 'Corrupted' perhaps?
Let's take some other examples.
"but I am completely honest whenever possible, even if that honesty may hurt someone else's feelings."
Why are you completely honest whenever possible? Who cares? Who made honesty a virtue? It is, of course, but how did you come to know it is a virtue, even if it may be hurtful to someone?
Also, with that statement in mind and the objectivist framework, how can you possibly say, "If you hurt other people it will be a detriment to your long-term happiness, because they will likely hurt you in return."
So why be honest then? Wouldn't it be better to not hurt someone, no matter what the cost, out of fear that they might 'hurt you in return'? So by this statement here we are led to believe that honesty might be a bad idea, especially if it 'hurts someone's feelings', cuz they might hurt you in turn. But you just spoke as though it were good to be honest even to the point of hurting someone's feelings!
You are right that the atheist response to the 'premise' that people can't fly is to 'invent the airplane' but as airplanes still crash and people still get hurt (and die) and people still lie and we agree that lying is not good but honesty is, etc, etc, does such a contraption really escape the problem?
In my mind, saying that 'atheists invented the airplane' in response to the demonstrably evident fact that people are 'bad' is only a patch job. The Christian worldview says that 'people can't fly' too, but don't believe that man-made solutions like 'airplanes' are much good, since people still go splat when they hit the ground. Keeping the analogy, Christians also have a proposal for how people might yet 'fly' (ie, be sinless) but argue that the operation must be performed by God.
I say that just to set the record straight that atheists don't occupy any kind of high ground by 'inventing the airplane' as though Christianity had no answer to the human condition. Christianity does have an answer you just don't like it.