The doctrine of assurance is so important to the lives of Christians because it gets at the heart of the nature of salvation and of God's character. Our war with sin has been won already, but in the battles left to fight until we're glorified, Christians often get discouraged. One of the most unfortunate and darkest ends of this discouragement is when one believes that they are no longer under the grace that saved them in the first place. This is a horrible predicament, because they are left in a position where they still believe in the existence of God and his justice, but now feel they are under his judgment. This obviously can be completely debilitating in every area of life.