We shall suppose, that a creature, possessed of reason, but unacquainted with human nature, deliberates with himself what RULES of justice or property would best promote public interest, and establish peace and security among mankind: His most obvious thought would be, to assign the largest possessions to the most extensive virtue, and give every one the power of doing good, proportioned to his inclination. In a perfect theocracy, where a being, infinitely intelligent, governs by particular volitions, this rule would certainly have place, and might serve to the wisest purposes: But were mankind to execute such a law; so great is the uncertainty of merit, both from its natural obscurity, and from the self-conceit of each individual, that no determinate rule of conduct would ever result from it; and the total dissolution of society must be the immediate consequence. Fanatics may suppose, that dominion is founded on grace, and that saints alone inherit the earth; but the civil magistrate very justly puts these sublime theorists on the same footing with common robbers, and teaches them by the severest discipline, that a rule, which, in speculation, may seem the most advantageous to society, may yet be found, in practice, totally pernicious and destructive.
Hume gives as historical examples of this type of religious fanatic some of the more extreme Puritans during the English civil wars; he also gives the Levellers as possibly an example of a similar group of political fanatics.
This passage always strikes me as a good example of Hume's dry wit.
[Quotation from An Enquiry Concerning the Principles of Morals, Section III, Part II.]