This is the topic of debate in chapter 3 of God or Godless?
Randal Rouser opens by imagining the mind-set of someone who tortures and kills people for fun. He imagines the character denying that what he does is evil, because (on the assumption that there is no God) morality is a purely subjective concept, with no objective basis for deciding whose morality is right and whose is wrong.
John Loftus, in his opener, says that the title of the chapter assumes that there is one God, whereas there are multiple concepts of God, with their own moralities. And the morality laid down by the Christian God does not have a very good track record, given the record of the Bible and the history of the church. Many non-Christian cultures have done very well for themselves without any Christian influence. Morality evolves (says Loftus) over time, in order to maintain a reasonably happy and stable society. The only objective morality available to us is that which we share as part of our common humanity. And, lest it be claimed that the existence of morality provides evidence for the existence of God, Richard Swinburne (“one of the greatest living Christian apologists”) is not convinced.
In my opinion, this topic needed a much greater depth of discussion than the format of God or Godless? permitted. However, I can agree with Randal that we should all be able to agree that some things (such as torture for fun) are plain wrong, and that it is unsatisfactory to explain why we think that they are wrong merely in terms of evolving social norms.
John, notwithstanding his protestations (“How long will Christians keep claiming this…?”) seems to have misunderstood the issue. Christians do not suggest either that their own behaviour is impeccable, or that non-Christians cannot behave ethically. The argument from morality is simply this: that most of us (Christians and non-Christians alike) agree that there is an objective basis for morality; Christians can posit a plausible source for this morality (namely, God), whereas atheists can posit no plausible source for it.
By the way, it’s worth nothing that when John cites Richard Swinburne as doubting the validity of the argument from morality, he is himself using one of the tactics – the argument from authority – that is most despised by many atheists. Presumably, he’s happy to label Swinburne as “one of the greatest living Christian apologists” because, on this point, he happens to agree with him.