The Cult of the V8 is the automobile and engine based Wasteland religion of Immortan Joe's War Boys.
The society of The Citadel, especially the War Boys, worshiped V8 engines as symbols of power and use them in the automobiles they build and drive.
Immortan Joe acted as the patriarchal and immortal god figure within the Cult of V8 and was the only one to grant access to the afterlife Valhalla. After Joe, divine power and influence was given to the All-Mother, who saw fit to allow the War Boys to continue their worship of all things mechanical.
There is a belief that the V8 engine is the source of the War Boy's strength, a power conduit between the world of the Wasteland and the afterlife - Valhalla, with guzzolene powering its wrath.
Those within the Cult of V8 referred to themselves as "half-lives", and acknowledge that their lives are short, caring only to serve Immortan Joe, and now the Vuvalini. "The Witness" is a ritualistic statement called upon when a War Boy plans to die in combat, and is intended as a sign of respect by other War Boys. War Boys despise the idea of "dying soft", and prefer to go out sprayed with chrome on the "fury road" to Valhalla.