On the relationship between god and morality, I described two views consistent with the proposition that there can be no morality without god.
One could adopt the view that this is like saying, "There can be no trees without god." While somebody can believe this, the belief is consistent with the view that a person does not need to believe in a god to know the height, mass, and age of a tree. These properties are truly objective - available to anyone regardless of their beliefs about what is necessary for their existence.
However, many theists do not adopt this view. They adopt an alterntive view that holds that a belief in and acceptance of god is necessary to be properly aware of moral properties. While they do not argue about the need to convert people in order to make them aware of the height, mass, and age of trees, it is supposedly necessary to convert people to make them properly aware of the moral properties of rape, murder, and theft.
Why is there this difference?
This difference is made more questionable by the fact that it is not the case that an atheist views all arrangements of matter and energy as qualitatively identical. Atheists can feal pain. The experience of pain does not require a belief in god - just like the experience of height, mass, and age. This shows beyond any reasonable doubt that it is not the case that whatever "value" is, it is not knowable only by those who believe in a god.
Yet, in spite of these obvious facts, we are still hold that a person must be converted to a particular religion to be properly aware of moral properties - even though, at the same time, we are told that they are objective and real.
Again, what motivates people to adopt this view rather than its alternatives?
The widespread acceptance of this view is easy to explain by appeal to a common human phenomenon of tribal chauvinism - a human disposition to want to see members of one's own tribe (including oneself) as morally superior to others. It feels good to think that "we" are better than "them". What feels good is mistaken for what feels right or feels true. Logic and evidence are cast aside in favor of the feel-good belief in tribal superiority.
This is a common phenomenon. We routinely see tribes adopting beliefs in the absence of reason or evidence whereby they hold members of a competing tribe as beneath them. Slavery was justified on the cherished beliefs that blacks were a child-like species better off in the household of a paternal slave master who exchanged his care and provision for their needs for a few hours of labor in the field. It can be found in the examples of "separate but equal" where provisions for "whites" and "negroes" sat side by side - separate and definitely not equal. Women were denied the right to vote and treated as property in part by the widespread belief that they were too emotional to make rational decisions. They, too, required the care of paternalistic figures who looked after their welfare and had a right to command absolute obedience, as if they were young children.
These were not simple mistakes. They were mistakes that served a political and social end. The lack of evidence or reason to support them was ignored because they "felt true" - and they "felt true" because they supported the conclusion that the members of the tribe that adopted them were superior to the others they imposed their will upon.
It would be a mistake to think that atheists are immune to tribal chauvinism. It is a part of our human nature.
As an example, there is a popular quote among atheists from Stephen Weinberg that says:
Religion is an insult to human dignity. With or without it you would have good people doing good things and evil people doing evil things. But for good people to do evil things, that takes religion.
It ignores the very conspicuous observation that an atheist can adopt a secular philosophy - such as Ayn Rand objectivism, Marxism, or Social Darwinism - that is just as prone to cause "good people" to do bad things as any religion. It ignores the fact that nothing that we find in religion was put there by God. Everything we find in religion was put their by humans with no divine guidance at all. It represents what people are inclined to believe in the absence of evidence. There is no evil that can be put into a religion that cannot be put into a secular philosophy that makes no reference to a god, if people want to see it there.
The tribal chauvinist is not after truth supported by reason and evidence. He is after a belief that allows him to claim the moral superiority of his tribe - regardless of reason or evidence. The atheist tribe is no different.
However, the fact that this is a human problem and not a "religion" problem does not change the fact that it is a problem. It may be a part of our nature - but it is a part of our nature that is responsible for great injustices and harm. It is a part of our nature that we are well advised to battle against when it appears in the atheist community as much as when it appears in religious doctrine. That a certain type of behavior can be explained does not imply that it is behavior that can be excused.
We have here, in the widely expressed version of the view that morality requires a god, a view motivated by tribal chauvinism that allows one tribe to view itself as morally superior to - "above" - another, and thus holding greater entitlement to the opportunities and benefits of civilized society. The view is not supported by any type of reason or evidence. It's supported by the "good feeling" of prejudice and bigotry - and as such it justifies nothing.