“Either God or the ego is insane” - ACIM
ACIM says there are ultimately only two internally consistent interpretations of reality.
The first says: God is Creator. Love is reality. Creation is extension. Unity is real. You remain as God created you.
The other says: You created yourself. Separation is real. Fear and attack are justified. Identity is private. You can oppose God. Death is ultimately real.
These cannot be reconciled. This is why it says: “partial allegiance is impossible.”
If the ego is right, God is wrong. If God is right, the ego rests upon an impossible premise.