Green’s Dictionary of Slang

(US) a gullible person, who will believe whatever they are told; thus make a believer (out of), to convince.

1925		D. Hammett ‘Corkscrew’ Story Omnibus (1966) 220: I picked the sawn-off shotgun out of its nest. ‘Can I borrow this? I want to make a believer out of a guy.’.
1979		E. Torres After Hours 248: Kleinfeld’s tanned hide would make believers of all the other corrupt bastards.
1989		P. Munro Sl. U.
1999		‘Master Pimp’ Pimp’s Rap 191: Now make them counterfeit niggers a believer.