Comte point of departure is that the living are always and everywhere governed by the dead. Almost nothing a person uses, thinks with or lives inside was made by anyone now alive: the language, the tools, the laws, the accumulated knowledge and the habits of cooperation are all inherited. The Great Being is that inheritance considered as a single continuing entity, of which the currently living are a thin surface layer.
Membership is earned rather than automatic. Comte held that the Great Being is composed of those whose contribution continues to act beneficially, which excludes both the merely harmful and the merely inert, and includes the unborn as its future constituents. That gives the doctrine its moral edge, since a person becomes part of the Great Being by what they leave behind and not by having existed.
From this follows what Comte called subjective immortality. There is no afterlife in the doctrine, and a person persists only in the memory and the effects they leave in the living, which is why commemoration is treated as a religious duty rather than as sentiment. The system adds two further terms to make a trinity of a kind: the Great Fetish, which is the Earth, and the Great Medium, which is the space in which everything happens. Neither is a deity and neither is worshipped in the way the Great Being is, and both exist in the scheme to give the object of reverence a setting.