Religious type initially essential to churches (5,8)
I believe the answer is:
roman catholic
'religious' is the definition.
The answer and definition can be both people as well as being singular nouns.
Maybe you can see a link between them that I don't see?
'type initially essential to churches' is the wordplay.
'type initially' becomes 'r' (I can't justify this - if you can you should give a lot more credence to this answer).
'essential to' means to look at the middle letters.
'churches' becomes 'Roman Catholics' (Roman Catholic is an example).
The middle of 'romancatholics' is 'omancatholic'.
'r'+'omancatholic'='ROMAN CATHOLIC'
Can you help me to learn more?
(Other definitions for roman catholic that I've seen before include "Its head is the Pope" , "RC in full" , "Religious believer" , "Church member" .)