Quote from ACIM Text Chapter 6, Section 8 - To Have Peace, Teach Peace to Learn It.

There can be no conflict between sanity and insanity. Only one is true, and therefore only one is real. The ego tries to persuade you that it is up to you to decide which voice is true, but the Holy Spirit teaches you that truth was created by God, and your decision cannot change it. As you begin to realize the quiet power of the Holy Spirit's Voice, and Its perfect consistency, it must dawn on your mind that you are trying to undo a decision that was irrevocably made for you. That is why I suggested before that you remind yourself to allow the Holy Spirit to decide for God for you.