But it is possible to have a "higher moral code" which is not based on belief in any divinity yet is still not just a personal opinion. Their adherence to the idea of 'doing unto others as they would have done unto themselves' is just derived from the utilitarian imperative, is all. Many atheists & agnostics simply substitute "God compels me to be good" with "The greater good & my own self-interest compels me to be good." They say:" I will aspire to the higher ideals of being ethical & honest & charitable & moral & teach my kids to live similarly because that promotes a better, safer & more prosperous & beneficial world for me & the ones I love."
Now I will say that non-believers who believe in the utilitarian 'commandment' to behave themselves for the greater good also really only have the luxury of subscribing to that philosophy because they're living for all practical purposes in a world where most of their fellow citizens have learned to live the same way through divinity-based morality. They should try going to a country where judeo-christian principles have never taken root at all some time & see how their pragmatically-inspired, secular philosphically-based moral idealism holds up in practical reality there.
I'm neither atheist nor protestant,yet as a cradle RC I certainly appreciate that my ability to enjoy the benefits of living in the most propserous, productive & stable society the world has ever known while still retaining a good deal of individual liberty is due in no small part to the Calvinist Christian principles of thrift, hard work, self-sacrifice & deferred gratification upon which this country was founded & to some extent still thrives, even if increasingly in only an atavistic way.
I'm only too painfully aware, for example, that I'd never have been able to enjoy the benefits of the relative order, safety & prosperity and personal freedom which emanate from the protestant-based civil ethic here in a predominately RC country. Spain? Italy? Even France? I love them all very very dearly, but HA!
And in a society where no respect for divine power has ever held sway at all? Fugeddaboutit completely! We'd be too busy trying to survive to ever have the luxury of imagining a library with books written by any one like J. Stuart Mill let alone having the luxury of adopting them as our secular philosphical ideal.
You misunderstand what motivates the faithful. It is not that God is "compelling" us to behave a certain way, but that Good and Evil are already defined by Him and we choose which we will promote.
Otherwise, it really just is opinion.