Telika: Secular maggots coopt history and munch on "hero" corpses just the same, making them symbolic figures of this and that, sacralizing them their own way, and planting their flags on them.
IAmSinistar: While I understand what you are trying to get at, and agree with some of your core reasoning about the commonality of root causation of affect, I think it's also seductive to overapply reductive parallelism between discrete magisteria.
Since systems like religion and secularism both come out humans, naturally there will be manifestations in each that are driven by the same causes. So it is easy to point to both and say "see, here one does X and the other does Y, naming them different but at heart being the same". But this is also facile if one doesn't consider the multifarious layers that go into those manifestations. This is where reductionism leads to oversimplification.
In terms of sociodynamics, I would say that the overarching difference between secularism and religion is that the former strives for universality, whereas the latter strives for segregation. Secularism is about seeing where we overlap and finding what we all want as humans. Religion, conversely, is about binary absolutes - the believer versus the doubter, the obedient versus the heretic, the saved versus the damned.
If you prefer, you can view secularism as striving for the minimal set, the baseline for what we must do as a society to ensure that individual rights and freedoms are honoured. Conversely religion strives for the maximal set, an orthodoxy of what every individual must believe, and must not believe, in order to ensure stricture. Secularism embraces heterogeneity, whereas religion espouses homogeneity.
A thought experiment that I often return to helps delineate this primary contrast: If we all woke up tomorrow as secular humanists, by the end of the day we'd all still be secular humanists. But if we all woke up tomorrow with the same religion, by the end of the day we'd have splintered into dozens of new sects. Because the former is predicated on where we are alike, and the latter on where we differ.
I am not sure of that, but it may be because I do not see secularism as a thing by itself. When I mention the secular hijacking of the dead, I have in mind the various ideological splinters that do not depend on religion : nationalisms, political currents, etc. We do not require religions for wars, bigotry and opposed identities. We have many secular forms of these. At the end of the day, in your thought experiment, we might still be at each other throats, even if our differences won't take a strictly religious form - other mystiques are available, other Great Ideas, abstract ideals, or even Ancestors to honor, who died for us, Founding Fathers whose Words are Golden. Stalinism and Hitlerism were pretty secular, yet (in similar and different ways) vastly mythological.
The legitimacy of simplifications depend on what aspect (or function) you wish to consider, and, yes, you can turn your attention to the differences if they pertain to the aspect that interest you, or if they play a role that was too dismissed in another. But do secular political ideologies function differently from religions when it comes to opposing people around essentialized identities, providing mythified role models, justifying the sacrifice of the self (and the other) for an abstract future often beyond one's lifetime, imposing an ethnocentered set of norms/values outside of which is only obscurantism, structuring time around intensification rituals, and socially stigmatising blasphemy ?
Are the differences between religiosity and secularism really relevant, there ? Or are religions a handy way for secularisms to present themselves as humanistic and rationnal ? Alternately, don't certain religious beliefs, just like certain secular systems, present more enlightened, progressive and "oecumenic" projects than some (both secular and religious) others ?