be under the delusion—
1. hold a false opinion or belief:
- Anyone who is under the delusion that corrupt governments are only in the Third World has better think again.
2. hold a belief that may be a symptom of madness:
- He was charged with shoplifting but the man was under the delusion that the supermarket belonged to him.
be under the illusion— = be under the delusion 1:
- Often our world is under the illusion that power comes in the form of money.