The wrongheaded notion that theists attempt to propagate is that concepts of morality, moral behavior, and understanding the difference between right and wrong come from ‘god’ – which is nonsense given the fact that there is no ‘god’ as perceived by theists.
And given the fact that there is no ‘god’ as perceived by theists, the fact that religion and ‘god’ are creations of man, so too is it a fact that man created concepts of morality, moral behavior, and understanding the difference between right and wrong independent of religious dogma – that those man-created moral codes are incorporated into religious dogma makes no difference.
Consequently, those free from religion follow the same moral codes as theists.