One believing article is true at first (6)
I believe the answer is:
theist
'one believing' is the definition.
(theists believe in a god or gods)
'article is true at first' is the wordplay.
'article' becomes 'the' (the definite article).
'at first' suggests taking the first letters.
The first letter of 'true' is 't'.
'the'+'is'+'t'='THEIST'
(Other definitions for theist that I've seen before include "Tithes for believer in God" , "Person in good faith" , "Believer in a god or gods" , "Person who believes in the existence of a god" , "Believer in god/gods" .)