The first believer in god (6)
I believe the answer is:
theist
'believer in god' is the definition.
(I've seen this before)
'the first' is the wordplay.
'first' becomes 'ist' (ist resembles 1st).
'the'+'ist'='THEIST'
(Other definitions for theist that I've seen before include "Person in good faith" , "Believer in god/gods" , "Tithes for believer in God" , "One who believe in a god" , "Religious type" .)