In C.S. Lewis' writing Man or Rabbit he answers the question of whether or not a person can lead a good life without being a Christian. Lewis seems to think this is a stupid question. He does not think anyone could simply want to live a good life without caring about believing anything. If a person has no set of beliefs how do they know what is right or wrong? A person has to believe in something to consider the life they have lead a good or a bad one.
Lewis raises the point that leading a good life is just going through the motions if it is not for an end goal. As Christians we believe that we are made for a better place and this life is only a means to an end. An atheist leads a life of no meaning, because according to them everyone lives for around eighty years and then their life simply comes to an end. Without God there is no purpose to any of our lives.
Lewis also points out that without God's help none of us can be moral. We are all fallen beings and without God's help we would all dwell in our sin. God helps us and brings us up since he is the ultimate good in the universe.