Cornelius Plantinga Jr. started off his book by going over the process of creation. In this chapter Plantinga goes over the inevitable next step, the fall. Plantinga explains that God created the world as purely good, but it is because of us that there is evil in it.
Plantinga talks about the fact that the world is not just divided between good people and evil people. Evil is a part of all of us; it is in all of our souls. The battle between good and evil is more complicated than a war between angels and demons. If God were to purge the world of evil at the present being he would divide every person against himself. Each person has a battle between good and evil fighting within.
Plantinga defines evil as anything that messes with shalom. So anything that disturbs the way God wants things do be is defined as evil. Plantinga explains that there is a distinct difference between sin and evil. Sin is the human act of doing evil. Sin has to be evil, but evil does not have to be sin.
Plantinga says that evil is a parasite. It cannot live without a host. Each of us harbors sin within our souls and we have the option to destroy it. Since we are fallen being none of us has conquered this demon within us except Jesus Christ and it is through his grace that we are saved.