Bantu is a linguistic classification and not an ethnic one, and the distinction has to be made first because it has repeatedly been ignored. The family holds something like five hundred languages spoken across most of Africa south of a line from Cameroon to Kenya, among them Swahili, Zulu, Shona, Kikongo, Luganda, Kinyarwanda and Xhosa, and the peoples speaking them are as different from one another as any set of peoples anywhere. The survey edited by Derek Nurse and Gerard Philippson is the standard reference.
The name comes from the widespread root for people, which appears with a class prefix as bantu, and it was coined by a nineteenth century philologist. In South Africa the apartheid state then took the word as an official ethnic label, which is why it carries an offensive charge there while remaining the ordinary neutral term in linguistics elsewhere. The atlas uses it only in the linguistic sense.
Religiously the family matters because a shared vocabulary travels with it. Terms for a high god, for the ancestors, for a spirit that possesses a person, for medicine and for the specialist who handles it recur across the family in cognate forms, which is evidence of a common inheritance of words rather than of a single religion.