"For if there is no ultimate justice then all of their moralizing would unintelligible because they have no point of reference... no objective standard to which they can appeal"
Of course, they could just do what you do, invent one.
So you see their objective point of reference is a globular star-cluster out on the very limits of the visible universe that encodes an absolute standard of right and wrong in the motions of its stars.
No, I don't have any photos of it.
No, no-one has ever actually measured those motions.
No, the method by which motions encode morality isn't explained.
No, I don't know how this globular cluster of morality came to be.
But I've got a book written by men who explain that were we able to see, measure and decode those moral absolutes, they would read as follows. Plus I have deeply-held beliefs about what morals would be writ there if I could see and read them, transmitted to my brain through the medium of gravitational waves.
There, a completely objective and secular standard of morality on which to base my arguments, wouldn't you agree?
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The religious standard of morality is no less subjective than any secularists, it is based on human interpretation of human writings. Just claiming that these writings derive from some objective source that can't be produced or checked is no different to a secularist claiming their objective standard was "written in the stars."