The first person who believes (6)
I believe the answer is:
theist
'person who believes' is the definition.
(someone who believes in a god)
'the first' is the wordplay.
'first' becomes 'ist' (ist resembles 1st).
'the'+'ist'='THEIST'
(Other definitions for theist that I've seen before include "One who believe in a god" , "Believer in the existence fo a god" , "Believer in a god or gods" , "Person in good faith" , "Believer in a creator god" .)