To me, Shinto gets complicated when following the first commandment (10)
I believe the answer is:
monotheist
'to me shinto gets complicated' is the definition.
I can't tell whether this definition defines the answer.
'following the first commandment' is the wordplay.
'following' says to put letters next to each other.
'first' becomes 'ist' (ist resembles mst).
'commandment' becomes 'mono' (I am not sure about this - if you are sure you should believe this answer much more).
'the'+'ist'='theist'
'theist' after 'mono' is 'MONOTHEIST'.
'when' is the link.
Can you help me to learn more?
(Other definitions for monotheist that I've seen before include "Christian, for one" , "One who believes" , "I'm A Believer" , "Maybe Christian" .)