Full Idea
At first men had no other kings but gods, and no other government than a theocratic one. ....By the mere fact that a god was placed at the head of every political society, it followed that there were as many gods as there were peoples.
Gist of Idea
In early theocracies the god was the king, and there were as many gods as nations
Source
Jean-Jacques Rousseau (The Social Contract (tr Cress) [1762], IV.8)
A Reaction
He must be thinking of the Old Testament histories here. (see Spinoza on that!). He says that the modern idea that these were all really the same god is ridiculous.
Book Reference
Rousseau,Jean-Jacques: 'The Basic Political Writings', ed/tr. Cress,Donald A. [Hackett 1987], p.220