The most common and most likely answer to this crossword clue is the 15 letter word PRESBYTERIANISM
I've seen this crossword clue in Independent
P
R
E
S
B
Y
T
E
R
I
A
N
I
S
M
Buddy explains!

The straight part
We know that "faith" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case faith = presbyterianism.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 PRESBYTERIANISM mean?
PRESBYTERIANISM noun- That form of church government which invests presbyters with all spiritual power, and admits no prelates over them; also, the faith and polity of the Presbyterian churches, taken collectively.