ACIM makes it very clear that the impossible—the split from God—did not happen, that “we” are forever safe in God. This makes no sense to the ego, because it doesn’t experience itself as being in God. But the ego itself isn’t an actual entity; it has no existence. Ego is nothing more than the belief of a thought that separation has occurred.
If it’s only a thought, what believes this thought?
That which calls itself “I” believes the thought. And the activity that results from believing the thought to be true is called ego.
So if I’m still safe in God, why don’t I feel so?
Because of the belief in the thought of separation.
The mind is a limitation of God’s Mind. God is pure subjectivity, and pure subjectivity is without objects—it contains all possibilities, and knows no thing. In order to know a thing—an object—God must seem to limit Itself into a subject. That activity of seeming to limit Itself creates an apparent limitation. The apparent limitation believing itself to be separate is what is referred to by “the separation”. No separation has actually taken place; God has simply seemed to limit Itself that it might know objective experience.
That limiting gives rise to the mind, which is the means to experience duality, or objective experience. The “mistake” is believing the thought that separation has actually occurred, that the world could actually be outside the “I”, and that the “I” could be outside God.
From this mistaken belief arises the world now experienced.