Extreme letters introduced to cunningly ensnare early Christians (9)

'early christians' is the definition.
(early Christian sect)

'extreme letters introduced to cunningly ensnare' is the wordplay.
'extreme letters' becomes 'AZ' (first and last letters of the alphabet).
'introduced to' means one lot of letters goes inside another.
'cunningly' indicates anagramming the letters (a cunning or tricky form of the letters).
'ensnare' is an anagram of 'narenes'.
'az' placed into 'narenes' is 'NAZARENES'.