The most common and most likely answer to this crossword clue is the 9 letter word UNITARIAN
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More clues leading to the result UNITARIAN
Buddy explains!

The straight part
We know that "church member" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case church member = unitarian.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 UNITARIAN mean?
UNITARIAN noun- One who denies the doctrine of the Trinity, believing that God exists only in one person; a unipersonalist.
- A Muwahhid.
- One who rejects the principle of dualism.
- A monotheist.
- Espousing a unitary view of something