Belief that won't change on reflection
I believe the answer is:
tenet
I believe this is a double definition.
'belief' is the first definition.
('tenet' can be a synonym of 'belief')
'won't change on reflection' is the second definition.
'that' acts as a link.
(Other definitions for tenet that I've seen before include "Doctrine, dogma, belief" , "Adiaphoron" , "Rule" , "Canon" , "Doctrine held to be true" .)