First of all I use the term *secular* lightly here as it's the only way in my pea size brain to encompass all worldviews that deny a transcendent source for morality or moral truths. So if it doesn't comport 1:1 please forgive me. Now to the post
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If the acceptability of your moral actions is determined by the society around you then you can't say that other people's moral stances are wrong. Only that it is wrong for our current society, according to our arbitrary preferences. In order to believe certain morals are correct then you need to have a reason as to why that’s the case, under naturalism or materialism, a philosophical perspective that denies the transcendent it’s impossible. You have no reason to be empathetic, you have no reason to survive or live, you have no inherent value. It's all arbitrary now. According to the modern secular worldview the accusations laid against Christianity have no weight because in the worldview that slings them there's no ability to claim moral truth, only personal preferences (based on experience of stimulus).
On top of this is the idea that social dysfunction (blanket term for the basis & justification for evolutionary morality) = a specific thing is a matter of subjective interpretation. For example the Romans who created the longest lasting empire waged warfare continuously. Something that would be seen as obscene today. If your morality is based upon practicality and what works, then a society who's economic basis is built upon slavery (The Romans) would view slavery as good. In order to call the Romans wrong for building their empire on the backs of slaves you would need a reason as to why slavery is wrong both for them and us. In which case in order to do so you would assume a moral standard, something not relative.
Lastly, in ANY *secular* worldview agreements upon behaviour =/= why I should follow them, they are arbitrary agreements and need a basis if you want to condemn someone. If a society comes together and agrees that torture for x reason is moral, and I disagree, which one would be right and why? The moral principles are entirely arbitrary, you need a reason as to why hurting people is bad and then a reason as to why it's true. If you have no reason as to why it's true then you literally have no reason to believe it or follow it, let alone legislate it. If your reason is the avoidance of harm, you have now assumed that the avoidance of harm is a moral truth and you assume the value of human life which has no basis in materialism & naturalism (the dogmas which *secular* science is dependent upon). It seems incoherent. The only consistent stance within the *secular* worldview is that morality is arbitrary preference. I've only ever seen one person take this stance and it was The Amazing Atheist (used to watch him back in my atheist days) and if you were to take it I couldn't argue against it, I could only disagree on other basis's like philosophy & etc.
[Edit: There seems to be misunderstanding, I don't know if that's because of a lack of clarity on my part or if it's because of the opposing view's presuppositions imparting blinders on those who think them (all presuppositions do in a sense when you explain from them frequently). So I thought it prudent to link a detailed comment/response that explains the points a bit further and might provide some clarification. This is it here.]
