Quote from ACIM Text Chapter 8, Section 1 - The Direction of the Curriculum.

The total senselessness of such a curriculum must be fully recognized before a real change in direction becomes possible. You cannot learn simultaneously from two teachers who are in total disagreement about everything. Their joint curriculum presents an impossible learning task. They are teaching you entirely different things in entirely different ways, which might be possible except that both are teaching you about yourself. Your reality is unaffected by both, but if you listen to both, your mind will be split about what your reality is.