In this writing C.S. Lewis addresses the idea that people only want to get into heaven because of the reward that waits for us there. He gives the example that money is not the natural reward for love so if someone recieved money for loving somone else they would be considered a mercenary, but if someone gets married because they love someone the reward is considered acceptable.
I am sure that there a people that are motivated to be a Christian because of the promise of eternal life. There also probably people who are Christians because of the fear of being forever damned, but these rewards and punishments are fitting.
Lewis explains that if we have a reward waiting for us in heaven we do not know that we have that reward waiting for us. We can never know if we are going to heaven or hell, so the fact that there is a reward is irelevent. A good Christian will lead his whole life following the teachings of the Bible not knowing if they are going to heaven or hell. Only at the end of our life can we truly know what awaits for us in the after life.
Lewis goes on to say that the fact that humans desire a paradise after death means that there probably is one. He uses the example of hunger. If a person feels hunger it does not nesicarily mean their hunger will be satisfied, but it does mean that they probably come from a race of beings that reapairs their bodies with food. In the same way many people desire paradise. This does not mean that everyone will make it to paradise, but it probably means that it is possible to obtain paradise.