This is why I think that it is harmful to describe secular humanism as a mere "absence of religion." The ideas that power it are really very sophisticated, and they are hard to teach. Skepticism is really rooted in a Pyrrhonist system of
epoché, which is just a method of suspending judgment, that they had originally used to attain
ataraxia, which is similar to the state-of-mind that we attempt to reach using mindfulness training, and that entire system is, in my opinion, probably derived from pre-Vedic Indian philosphy. Wildly enough, we skeptics are arguably practicing a sort of pre-Vedic meditation system. As easy as it sounds to just not believe something just because it feels good to believe it, that's really easier said than done, and it takes most of us a substantial amount of practice.
That is not even the only component of secular philosophy. To say that we did not have an "organized religion" would be way off-the-mark. It is a very complex system of thought with many functioning parts. Trying to say that you can get a full system of philosophy by simply subtracting religion is actually kind of wacky.
Unfortunately, many of us secular humanists underestimate our own minds' capability of doing things that are even worse than anything that comes from religion. I would hold up the example of deconstructionism. It is not a religion, yet it kind of acts like one. Adherents to that ideology can be just as bad as any religious zealots. They preach their gospel to you with the assumption that you are going to have some sort of "Road to Damascus" moment, and the "scales will fall from your eyes." On one hand, it is theoretically a form of secular philosophy. On the other hand, it is a dumb one, and it has a supernaturalistic quality about it. There are many types of inaccuracies we can develop, in our thinking, besides just believing in a religion.
Rational thought is truly the prince with a thousand enemies.
Run, rabbit, run
Dig that hole, forget the sun
When at last the work is done
Don't sit down, it's time to dig another one
Long you live and high you fly
But only if you ride the tide
Balanced on the highest wave
Race toward an early grave
I suspect that what you are probably referring to,
@Unknown Soldier, is actually related to the old Stoic system of philosophy. It is a very old way of thinking. According to the ancient Stoics, we ought to treat people as equals, even if they are not equals. We should not care about the differences between people, even if they exist. Men and women might be born different, but the virtuous way to behave toward a human being is always the virtuous way to behave toward a human being. We do not treat them the same out of an incorrect belief that they are the same, but what constitutes virtuous behavior is always what constitutes virtuous behavior. If we let other people around us define us, then we lose control of who we are. It's about exercising an internal locus of control. This is really the core of egalitarian thinking.
Yes, people that happen to observe Christianity can use this set of ideas. It is secular philosophy, not atheist philosophy. There is a difference. It is just like saying that someone can be a Buddhist yet practice Taoism. Taoism is not a religion, but it is a philosophy that anybody can practice, including Buddhists or even Christians.
In fact, I would even argue that many aspects of secular philosophy might be even more widespread than Christianity. This might sound like a strange statement, but it makes more sense if you understand the concept that a Christian can also practice secular philosophy. Christians might practice it separately, or they might practice it as a form of syncretism, where they incorporate parts of secular philosophy into their own belief-system.
Even the idea of a social contract comes from Epicurean philosophy. The idea of the Constitution of the United States of America was probably an attempt by Thomas Jefferson to incorporate an Epicurean idea, regarding social contracts, into the development of a new independent state. Even though Thomas Jefferson was theoretically a Christian simply because you could not get anything, politically, done if you did not go to a church at some point, the cat actually loved the writings that he believed were correctly attributed to Epicurus.
I think that it would hurt our entire culture if we treated secular humanist philosophy as a sort of "He-Man Religion-Haters' Club: no theists allowed!" We ought to embrace the idea that anybody can understand these ideas, whether they are religious or not. Everybody can live better lives based off of these ideas.
Furthermore, I think we ought to embrace the idea that we represent an ancient and rich cultural heritage. Our cultural heritage is just as ancient and just as cool as that of any religion.