Apparently first to infiltrate these diminished believers? (7)

I believe the answer is:

theists

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'believers?' is the definition.
(theist is a kind of believer)

'apparently first to infiltrate these diminished' is the wordplay.
'apparently first' becomes 'ist' (ist resembles 1st. I am not sure about the 'apparently' bit.).
'to infiltrate' indicates putting letters inside.
'diminished' means to remove the last letter.
'these' with its last letter taken away is 'thes'.
'ist' put inside 'thes' is 'THEISTS'.

Can you help me to learn more?

(Other definitions for theists that I've seen before include "Believers" , "people who believe in God" , "religious people" .)

I've seen this clue in The Irish Times.

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