Christian: So, how do you know what is right and wrong? How do you decide if something is right or if something is wrong?

Atheist: If it hurts others it is wrong.

Christian: Why should I care if what I do hurts someone else?

Atheist: Because it is wrong to hurt someone else.

Christian: You just went in a circle and ended up back where you started.

God is the standard of right and wrong.  Without Him we have no idea of right and wrong.  ‘The fool says in his heart, “There is no God.” They are corrupt, and their ways are vile; there is no one who does good.’ Psalm 53:1