Episode 8: Why is sin bad and purity good?

Tom, Karen and Matt are finally back in (well, outside) a pub as they return for the next episode. They are exploring why does God/the Bible say some things are sin, and other things are good? Are there any reasons for these things, or is it just something God made up - or the writers of the Bible made up? Aren't most of these ideas out of date?If you want to be clever, you could say they are exploring the origin of ethics.