The following can indicate a delusion:
The patient expresses an idea or belief with unusual persistence or force. Check!
That idea appears to exert an undue influence on the patient's life, and the way of life is often altered to an inexplicable extent. Yep!
The individual tends to be humorless and oversensitive, especially about the belief. Hmm...
An attempt to contradict the belief is likely to arouse an inappropriately strong emotional reaction, often with irritability and hostility. Ditto!
The patient is emotionally over-invested in the idea and it overwhelms other elements of their psyche. True!
The individual experiences a heightened sense of self-reference. Events which, to others, are nonsignificant are of enormous significance to him or her, and the atmosphere surrounding the delusions is highly charged. BINGO!