Bwiti is the religion most associated with this region outside it, practised among the Fang, the Mitsogo and neighbouring peoples of Gabon, where it has legal standing. It is a religion of initiation built around the root bark of the iboga shrub, taken in quantity once at initiation and in small amounts thereafter to sustain long night ceremonies of drumming, harp and song. Its cosmology absorbs Christian material without becoming Christian, and its congregations have their own officers, calendar and architecture.
James Fernandez's ethnography of Fang Bwiti is the standard extended study in English, and its value lies in what it declines to do. It does not present Bwiti as a survival of something older, and it does not reduce the initiation to the pharmacology of the root. It treats the religion as a contemporary response to a specific colonial situation by people making deliberate choices, which is both more respectful and more accurate than the alternatives.
What an ethnography cannot establish is also worth stating plainly. It cannot fix how old the tradition is, because the record does not reach back far enough and the tradition's own account of its origins is a religious account rather than a chronological one. It cannot speak for the initiated on matters they hold as restricted. And a single fieldworker's access, however long, is access to particular congregations at a particular time.
Iboga has since become an object of pharmaceutical and recreational interest elsewhere, which is not the same subject.