Evangelist (4)
'evangelist' is the definition.
(one of the four Christian evangelists)
This is all the clue.
(Other definitions for john that I've seen before include "& 19 across Traditional archetypal Englishman" , "Trick" , "Matthew, Mark, Luke and . . . ." , "With nineteen accross twentieth century US actor known for westerns, etc" , "From Gwen" .)
(Other definitions for luke that I've seen before include "Third NT book" , "Gospel writer" , "One of the four Evangelists" , "Physician gospeller" , "One of the Apostles" .)
'evangelist' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for apostle that I've seen before include "Principal champion" , "eg John, Paul" , "Teacher of new faith or movement" , "Maybe Peter" , "To lapse (anag) - ardent supporter of a cause" .)