Other people adopting one's beliefs (6)
I believe the answer is:
theism
'beliefs' is the definition.
(belief in a god or gods)
'other people adopting one's' is the wordplay.
'other people' becomes 'them' (those people over there).
'adopting' indicates putting letters inside (inserted letters are taken on or adopted).
'one's' becomes 'i's' (Roman numeral).
'them' enclosing 'is' is 'THEISM'.
(Other definitions for theism that I've seen before include "Belief in a deity" , "The doctrine or belief in the existence of a God or gods" , "Belief in a creator" , "of faith" , "Belief in existence of a God revealed to man" .)