Well you can tell them first hand,
Define what a good person is.
then tell them have you ever done anything bad once in your life? Have you ever lied, stole, cheated, covet, take God's name in vain, get jealous, or angry for no right reason, prideful, curse, hated, had premarital sex, immortality, disrespected their parents or forget God in your heart and take for yourself idols of pleasure?
Then ask them to define what a good person is again.