People provided with moral crutches and encouraged to use them have less need, and often less inclination, to make independent moral choices or even think about morality, than those provided with no moral rule book to lean on, who are forced to think about their actions and develop a personal moral philosophy.
Well, yes. Some of them. Some of them join a social sub-culture of an in effect in-group of rational people and develop their own moral code of being in effect rational.
You have to remember that not all moral codes are in books. So of them are shared through communication in an in-group. Learn your sociology of human groups.
And learn that not all moral codes are in books as you use the term.
The problem is that you can be level 3 on Kohlberg and an atheist.
Your assumptions is that all atheists are generally of an universal kind of cognition that they can be authentic as per level 5 on Kohlberg. That is not the case in practice and we have at least one atheist here, who can't do that. And we have some kind of in-group behavior for some of the atheists
The problem is that no humans is outside nature and nurture and that includes atheists.
So here is the joke. We are all humans and can be observed for our behavior and that includes you and I. And the general biology, sociology and psychology applies to us all and not just everybody else than you.
So you in effect should start observing all the atheists here and learn that they are not the same for morality in effect.