Quote from ACIM Text Chapter 7, Section 6 - From Vigilance to Peace.

The mind that accepts attack cannot love. That is because it believes it can destroy love, and therefore does not understand what love is. If it does not understand what love is, it cannot perceive itself as loving. This loses the awareness of being, induces feelings of unreality and results in utter confusion. Your thinking has done this because of its power, but your thinking can also save you from this because its power is not of your making. Your ability to direct your thinking as you choose is part of its power. If you do not believe you can do this you have denied the power of your thought, and thus rendered it powerless in your belief.