The most common and most likely answer to this crossword clue is the 15 letter word ECCLESIASTICISM
I've seen this crossword clue in The Times
E
C
C
L
E
S
I
A
S
T
I
C
I
S
M
Buddy explains!

The straight part
We know that "churchiness" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case churchiness = ecclesiasticism.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.
What does ECCLESIASTICISM mean?
ECCLESIASTICISM noun- Strong attachment to ecclesiastical customs and practices.