Not *just* human reason.
The basic idea is that moral systems are sets of rules and procedures... do this, don't do this.
"Good" ones help societies, and the people in them, thrive. "Bad" ones don't. This is how we call moralities "good" or "bad"... not *morally* good or bad, but good or bad for purpose.
How we arrive at those moralities isn't just pure reason, it's a variety of discovery techniques.
The first is biological evolution. This is why, for example, bees have a sense of property rights (anyone who disagrees is invited to test this by hitting a beehive with a stick).
This gives a set of basic behaviors that are built into us, instinctive. Because we are a pack animal, it govern not just us, but how we relate to the tribe.
And these are why we feel, very strongly, that we have an innate sense of right and wrong. Because we do. Theists believe it is built into the universe, by god. Atheists think it is built into us, by evolutionary processes, discarding what doesn't work and keeping what does.
The second way we arrive at moral precepts, which further refines what we get from biology, is cultural evolution.
Parents pass moral teachings to children. Societies with good ones thrive. Societies with bad ones don't. Hundreds of thousands of Aztecs were conquered by a few hundred Spaniards for a reason.
The third way is the deliberate investigation you speak of, which isn't just reason, its also empiricism.
That's what we are doing now.
These stages don't occur in a vacuum. They build on previous ones. All of our thoughts and investigations about morality are informed by our cultural traditions and our instinctive pack behavior.
To an atheist, Christians appear to see their moral laws as bypassing this whole process, a direct line to an alleged architect of the universe who gives you a definitive answer. A sort of "Christian exceptionalism", if you will.
We don't agree.
We see Christian morality simply as parts of the second and third stages... traditions that worked + deliberate moral reasoning.
Why don't we agree with Christian exceptionalism?
It's not just because we don't believe in the alleged source.
It's also because we have seen Christian morality undergo the process of cultural evolution (stage 2), and changes in its moral reasoning (stage 3) based upon those changes.
What a modern Christian, of any stripe, considers the morally imperative will of god only has the barest of overlap with what a medieval European Christian would believe. And neither would overlap much with an early Roman Christian.
Also, modern Christians don't completely overlap even with each other.
So, we don't buy that Christianity is the One True Word that ends the discussion. Because it isn't One.
And it certainly hasn't ended the discussion.
Now, you can argue that Christianity has arrived at a point where it is a positive influence. A case can certainly be made for that.
But if you argue that, then you are viewing Christianity through my lens, not yours... viewing is a step in moral evolution, not a process-ending divine revelation.
Which would kind of undercut your point.
The dichotomy isn't "Christianity bad" vs "Christianity good", or even "Christianity wrong" vs "Christianity correct".
It's "Christianity final answer from god" vs "Christianity part of ongoing moral debate".
As a side note, "Christianity part of ongoing moral debate" doesn't necessarily imply atheism. It seems to be that one could be a Christian, believe in a creator god, believe that god has given us some rules, but also that god has declined to give us all the answers and end the debate.
This would probably imply the belief that he wants the debate to happen.
I would say most atheists who give this problem considerable attention believe there exists a morality in the abstract that can be discovered through human reason. That this morality, this ethos, is intrinsically tied to the nature of reality. That these ethics can be sussed out, tested, and repeated with predictable results because they are part of the logos of the universe.
I would also say they believe this logos and ethos were not created, but sprang into being along with the cosmos.
Of course, the Christian conception of God, is he is the logos and ethos, amongst other things. Their existence in our universe is due to him creating the universe, a reflection of the divine, a part of the divine.
When Christians hear atheists talking about this, we hear you describing in part God, even if we disagree with some of the details.
There exists the “unexamined atheist” who views morality more as a social construct. A social tool to be crafted however a society sees fit. There exists no abstract objective moral difference between the Aztec human ritual sacrifice and the western conception of human rights. To them, whether they admit it or not, their god is power. He who can enforce his rule defines morality.