Jean Blum characterized Hamann’s thought as follows: “Hamann’s thought is what those who do not normally think would think if they did think.”

That gets it pretty well, as does Berlin’s comment that Hamann “was a major force in transforming the ideas which hitherto had lived in small, self-isolated religious communities, remote from and opposed to the great world, into weapons in the public arena.” (Berlin’s following sentence, “His was the first great shot in the battle of the romantic individualists against rationalism and totalitarianism,” is the kind of half-truth with which his book on Hamann is stuffed.)

In short, Hamann’s achievement was to present an intellectually sophisticated form of pietism. He might also be called the first populist philosopher.