Religion is a Delusion/Mental Illness

[Angrboda]

From what I understand it to be, having delusions, is having strong/fixed beliefs, despite overwhelming evidence to the contrary.

I would say not, because from their point of view, the evidence to the contrary is not overwhelming; their culture supplies them with positive reasons for belief. More than that, I'd say it's a difference in nuance. I knew my beliefs were delusional, yet I couldn't challenge or alter them. Religious extremists may be inflexible, but their beliefs aren't 'fixed' in the same sense. My beliefs were fixed because my brain hardware was malfunctioning; an extreme believer is inflexible because the totality of their experience tells them it's reasonable. It's a difference between believing for flawed reasons, and believing for no reasons at all.