The third chapter of Engaging God's World, "The Fall", discusses the affect of sin on our relationship with God. Plantinga explains human desire to "keep living our lives against what's good for us" and that our irrational decision to constantly disobey God has gone back to Adam and Eve and the first sin.
Plantinga describes sin as a deviation from the path of light and truth, which is God's path, into the darkness. As in Lewis' "The Poison of Subjectivism", Plantinga discusses that humans lack the rod with which to measure right against wrong. It is only by the grace of God and the belief of Jesus Christ as your savior can you know the difference between good and evil. However, by our original sin, "we have separated ourselves from God, who is our true life and have corrupted our entire nature."
In addition to corrupting ourselves when we sin, we corrupt others too. Plantinga gives the example of a father who beats his son, will eventually have a son who beats others and so on and so forth. Sin has a way of gaining momentum and in this momentum, sin is viewed as "normal." Viewing sin as something "normal" is such a danger to not only yourself, but something that is a threat to our society and eventually all mankind. For if a culture starts to fall prey to the corrupt, it will begin to lose hope. It is our duty, as Christians, to not allow this "normality of sin" ever occur. We must constantly fight for good and against evil in this world.