Source of light recognised at first in old Christian society (5)
I believe the answer is:
torch
'source of light' is the definition.
(a torch provides light)
'recognised at first in old christian society' is the wordplay.
'at first' indicates taking the first letters.
'in' is an insertion indicator.
'old christian society' becomes 'Toc H' (20th-century Christian organisation).
The initial letter of 'recognised' is 'r'.
'r' placed inside 'toch' is 'TORCH'.
(Other definitions for torch that I've seen before include "Portable light; burn" , "may get one glowing" , "Source of light" , "Set alight deliberately" , "Portable lamp; set alight" .)