Quote from ACIM Text Chapter 5, Section 5 - Teaching and Healing.

What the ego makes it keeps to itself, and so it is without strength. Its existence is unshared. It does not die; it was merely never born. Physical birth is not a beginning; it is a continuing. Everything that continues has already been born. It will increase as you are willing to return the unhealed part of your mind to the higher part, returning it undivided to creation. I have come to give you the foundation, so your own thoughts can make you really free. You have carried the burden of unshared ideas that are too weak to increase, but having made them you did not realize how to undo them. You cannot cancel out your past errors alone. They will not disappear from your mind without the Atonement, a remedy not of your making. The Atonement must be understood as a pure act of sharing. That is what I meant when I said it is possible even in this world to listen to one voice. If you are part of God and the Sonship is One, you cannot be limited to the self the ego sees.