The most common and most likely answer to this crossword clue is the 13 letter word PROTESTANTISM
I've seen this crossword clue in The Sunday Telegraph
P
R
O
T
E
S
T
A
N
T
I
S
M
Buddy explains!
The straight part
We know that "Part of Christianity" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case part of christianity = protestantism.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.