Quote from ACIM Text Chapter 4, Section 2 - Right Teaching and Right Learning.

Teaching and learning are your greatest strengths now, because they enable you to change your mind and help others to change theirs. Refusing to change your mind will not prove that the separation has not occurred. The dreamer who doubts the reality of his dream while he is still dreaming is not really healing his split mind. You dream of a separated ego and believe in a world that rests upon it. This is very real to you. You cannot undo it by not changing your mind about it. If you are willing to renounce the role of guardian of your thought system and open it to me, I will correct it very gently and lead you back to God.