Catholic somehow matches one in Christian principles (9)
I believe the answer is:
catechism
'catholic somehow matches one in christian principles' is the definition.
I can't judge whether this defines the answer.
'catholic somehow matches one in' is the wordplay.
'catholic' becomes 'C' (abbreviation as in RC for Roman Catholic).
'somehow' indicates anagramming the letters.
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'matches'+'i'='matchesi'
'matchesi' is an anagram of 'catehism'.
'c' placed into 'catehism' is 'CATECHISM'.
Can you help me to learn more?
(Other definitions for catechism that I've seen before include "Q&A session" , "Set of questions for religious teaching" , "Schematic for religious questions-and-answers book" , "Schematic for religious principles in question and answer format" , "Series of questions" .)