Quote from ACIM Text Chapter 2, Section 1 - The Origins of Separation.

All fear is ultimately reducible to the basic misperception that you have the ability to usurp the power of God. Of course, you neither can nor have been able to do this. Here is the real basis for your escape from fear. The escape is brought about by your acceptance of the Atonement, which enables you to realize that your errors never really occurred. Only after the deep sleep fell upon Adam could he experience nightmares. If a light is suddenly turned on while someone is dreaming a fearful dream, he may initially interpret the light itself as part of his dream and be afraid of it. However, when he awakens, the light is correctly perceived as the release from the dream, which is then no longer accorded reality. This release does not depend on illusions. The knowledge that illuminates not only sets you free, but also shows you clearly that you are free.