Quote from ACIM Text Chapter 6, Section 1 - Introduction.

You have been asked to take me as your model for learning, since an extreme example is a particularly helpful learning device. Everyone teaches, and teaches all the time. This is a responsibility you inevitably assume the moment you accept any premise at all, and no one can organize his life without some thought system. Once you have developed a thought system of any kind, you live by it and teach it. Your capacity for allegiance to a thought system may be misplaced, but it is still a form of faith and can be redirected.