My argument would be: "Do whatever I want? Then that would make me an absolute a moralist. But there are many things I do not want to do; nor do I choose to do them."
Then I would challenge them to be absolutely honest, at all times, especially about the Bible.
For example, I would ask them if they would obey every commandment given by the biblical god. Then I would read to them Deuteronomy 20:16-18 and ask: What kind of morality is that, and if they follow such a god?
Then I would read 21:18-21 and ask: When was the last time Bible believers followed this teaching?
What about Exodus 32:25-29? Was the Biblical god moral? Why did he exempt Aaron?
I would challenge them to believe ALL the Bible and compare what it says to the facts.
THE TRUTH? IN THE BIBLE
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The Bible is filled with scores of nonsense statements and contradictions to commonsense.
http://www.infidels.org/library/modern/jim_meritt/bible-contradictions.html