In Plantinga's first chapter he talks about two different types of desires: material and spiritual. Our desire for material and earthly things causes us to look for contentment in fleeting pleasures and individual moments of happiness. This does not bring joy like fulfillment of spiritual desires brings joy. Our spiritual desires are fulfilled by God and by doing his calling. The only way to fill the void in our lives is to fulfill our spiritual desires, and not our earthly desires.
Plantinga goes on to talk about how everyone longs and desires for God, whether they know it or not. I feel as though this statement has been misunderstood by quite a few people. Plantinga is by no means saying that humans are not totally depraved. He is also not saying that everyone will choose to follow God. I believe that he is saying that everyone has a void that they have a desire to fill. This void can only be filled by God. Therefore, whether people know it or not, they desire God. This does not mean that people will not try to fill the void with material things. It simply means that everyone needs God to fill the void.