At an individual level, I agree. But I stipulated "at a cultural level", by which I mean something like "if the culture is morally relativistic, I don't think you'll see a critical mass of principled individuals that would prop up a principled culture". I don't think moral relativism is compatible with a principled culture, at least not given the parameters of human psychology.
> No, people have always found a way to justify what want to be right regardless of the underlying ideology.
To be clear, I didn't claim moral absolutism generally or religion in particular is a panacea against this sort of self-dishonesty, but I think you see less of it and instances are less pronounced if only because that individual will lose credibility with his peers. In a morally relativistic culture, the offender only needs to argue "that's your morality, not mine" or "that's your truth, not mine", which is roughly what we see today.
> We've just become more explicit about it in some cases. People's beliefs about what is right tend to be very convenient for them regardless of what they claim their beliefs to be.
I think there's always a temptation to convince oneself that what they want is moral, but I think that temptation is inhibited because he knows his community won't let him get away with it (his reputation would be damaged if he advocated for a few overtly immoral things or many subtly immoral things, consistently). Further, with religions in particular, there is often a text that lays out the morality and there is only so much word lawyering someone can do without overtly running afoul of it. It further constrains the impulse. And with religions like Christianity in particular, a central activity is reflecting on one's selfishness and working against it (the concept of "dying to oneself").
Like I said before, it's not a panacea (there are many religious people and indeed Christians who subvert it), but I think it is a bulwark that we tear down at our own peril.