The most common and most likely answer to this crossword clue is the 23 letter word TRANSUBSTANTIATIONALIST
I've seen this crossword clue in The Times
T
R
A
N
S
U
B
S
T
A
N
T
I
A
T
I
O
N
A
L
I
S
T
Buddy explains!
The straight part
We know that "I hold traditional Catholic belief" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case i hold traditional catholic belief = transubstantiationalist.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.