I am not 'cutting corners', but this being an internet forum, I put forward some points in a summary form.
Looking back again at that phrase that I quoted from Vijay:
'I, an atheist, hereby assert that whether or not love and hate are real is categorically a matter of opinion' -
Looking at the context in which the statement was made, I see that this might have been intended ironically, whereas I read it at face value. So, I am willing to acknowledge it might have been misinterpretation on my part.
But reading it at face value, what I took it to mean is just what it says: that 'whether hate and love are real is a matter of opinion'. And I took that as an example of the view that ethical matters, generally, are a matter of opinion, or, perhaps, 'a matter of individual conscience'. And there are plenty of people who will say that they are exactly that. Contrast the traditionalist view (which is not exclusively Christian) that there are 'real virtues', that is, virtues which are intrinsically valuable, regardless of whether you or I agree that there are. (Aristotelean ethics are an example of that kind of 'virtue ethic'.)
Now the 'slippery slope' argument is as follows. If one rejects the traditionalist basis of ethical judgements, such as that encoded in the Jewish and Christian traditions, then you have to find some other basis for ethical values and judgements. There are other such bases, for instance utilitarianism, 'the greatest good for the greatest number'. Or you might say 'the Golden Rule' of 'do unto others...' suffices as the basis for an ethical system, as well it might.
But it is not hard to make the case that in the absence of a generally-accepted set of moral and ethical norms, that the viewpoint of 'moral relativism' and/or 'moral subjectivism' is a likely outcome. 'Moral relativism' generally rejects the idea of 'moral norms' or it might say they are matters of social convention, or are the product of a specific world-view; in any case, a moral relativist won't generally recognize that there are such things as 'moral facts' or 'laws'.
Consider this hypothetical: sometime in the not-too-distant future, in an over-crowded and resource-depleted planet, a nationalist political movement starts up which argues that, due to the dire circumstances in which humanity finds itself, it is necessary that support ought to be withdrawn from some populations, who are in any case not productive or not in a position to support themselves, so as to concentrate resources for the benefit of those populations who at least have some chance of creating a decent life.
Now the Christian response to that would be that it was out of the question, because
all people are equal. But that, in turn, is a value-judgement, even if it is one that we nowadays take for granted; but it is one of the aspects of what we now know as 'human rights' that was the product of Christian political systems.
Of course there are many things that could be discussed here. The point I was making, and I admit I made it in too peremptory a fashion to begin with, is that in a completely secular worldview, it is difficult or even impossible to find a point of reference for the idea of a 'moral law', and even a lot of what we take for granted under the umbrella term 'human rights'. Many of those concepts were grounded in the Christian philosophy of 'the person'.
You will notice that the existence and/or relevance of the concept of 'the person' is indeed under question by many of the materialist critics of Christianity; Daniel Dennett, for instance, says there is nothing wrong with acknowledging that human beings are really only 'moist robots'
1. He and others like him also says that the human mind is no different to a computer, and there is no difference in kind between humans and animals. So I see this kind of thinking as basically 'anti-humanist' - which is what is what I mean by a slippery slope.
Anyway on that note I really do have to sign out for a few days, I'm going somewhere with no internet connection.
