Robespierre believed that there was someone who was watching over France, and that was the Supreme Being. The cult represents an innovation in the "de-Christianization" of French society during the Revolution, in that Robespierre sought to move beyond simple agnosticism (often described as Voltairean by its adherents) to a new and, in his view, more rational devotion to the Godhead. (Compare the cult of Reason, advocated by Jacques Hébert and the enragés, and explicitly opposed to Robespierre's more theistic concept of the Supreme Being.)
Robespierre's proclamation of the cult as the new state religion in 1794 was possibly one of the factors that prompted the Thermidorian Reaction. The Cult of the Supreme Being was thus aborted instantly.
This new state religion by Robespierre did not allow freedom of religion, which angered the population of France because it contradicted the supposed ideals of freedom of the Revolution.