The irony here is that any number of religious folks will remind us that without God, it is not possible to wholly differentiate good from evil.
That, in other words, without a transcending [omniscient and omnipotent] point of view, there can be no teleological component available to mere mortals in a universe construed to be but a “brute facticity”.
Instead, one or another individual subscribing to one or another “Humanist” philosophy, will concoct one or another moral narrative that is said to reflect the most rational and virtuous frame of mind.
Just pick a context and a set of conflicting value judgments. And then with the right ideological/deontological assessment it can be determined which behaviors all rational men and women are obligated to embrace.
Just ask them.
And then in any particular community ensconced in any particular historical/cultural context a consensus can form around a set of prescribed and proscribed behaviors. Predicated on the assumption that right makes might.
On the other hand, the entire trajectory of human interactions over thousands of years puts the lie to this again and again and again. Instead, we have the same conflicting arguments regarding the same conflicting goods. Nothing ever really ever gets resolved.
And even regarding extreme behaviors like child abuse, rape, murder, slavery and genocide, there appear to be arguments available to those able to rationalize them. And what arguments are available able to obviate the motivation of the sociopaths/narcissists; those who see morality as revolving entirely around what sustains their own wants and needs.
That’s why some folks abandon moral narratives per se in order to insist that right and wrong can be understood “naturally”. Once you have grasped that the human species is just a natural extension of the evolution of life on earth, you need merely grasp that there are certain “biological imperatives” that underlie all human behaviors. And that, in turn, undermine all attempts by those who suggest that historical and cultural memes have a role to play as well.
In other words, anything [in a Godless universe] that allows them to insist that there is in fact a certainty to be found when values come into conflict. And they know this because, in fact, they have already found it.