Quote from ACIM Text Chapter 6, Section 4 - The Relinquishment of Attack.

As we have already emphasized, every idea begins in the mind of the thinker. Therefore, what extends from the mind is still in it, and from what it extends it knows itself. The word "knows" is correct here, because the Holy Spirit still holds knowledge safe in your mind through His impartial perception. By attacking nothing, He presents no barrier to the communication of God. Therefore, being is never threatened. Your Godlike mind can never be defiled. The ego never was and never will be part of it, but through the ego you can hear and teach and learn what is not true. You have taught yourself to believe that you are not what you are. You cannot teach what you have not learned, and what you teach you strengthen in yourself because you are sharing it. Every lesson you teach you are learning.