Quote from ACIM Text Chapter 2, Section 2 - The Atonement as Defense.

True denial is a powerful protective device. You can and should deny any belief that error can hurt you. This kind of denial is not a concealment but a correction. Your right mind depends on it. Denial of error is a strong defense of truth, but denial of truth results in miscreation, the projections of the ego. In the service of the right mind the denial of error frees the mind, and re-establishes the freedom of the will. When the will is really free it cannot miscreate, because it recognizes only truth.