Apologetics uses the moral argument for the existence of God because nothing except God can ground objective morality. Without God, all you have is people exercising subjective morality that suits their preferences. Add to that the fact that on atheism humanity has no free will, and you now have a contradiction. What kind of responsibility do we have toward each other and to whom are we ultimately responsible? Check out the article below that helps explain the impossibility of morality for atheists: