Do non-believers have a basis for saying some things cause harm? I think so. Just look at a burning child. We have a computer that computes the steps. Our brains. Such things cause harm. It's obvious. The basis for morality is empathy. The divine command theory has no room for it. If God is thought to command killing witches then empathy be damned. While there are two greatest commandments the only one that counts is the first one. Christians need not be concerned with the plight of human suffering, only loving the god in one's head.