Charles Taylor's Modern Social Imaginaries is concerned primarily with how modern society created three new spheres in which people could act: the economy, the public, and "the people." All three were created by imagining them as separate from, and a check on, the polity - the state, as we now think of it.
Premodern Europeans already thought of a sphere separate from the state which acted as a partial check on it: the church. The public sphere, Taylor suggests, is analogous to the church. But there is an important difference: the public sphere is secular. Taylor does not mean that the public sphere has to be separated from God, nor that the people acting in it must be separated from religion. Rather, he means that the public sphere is a place created in time by the people (some of them, anyway) acting collectively. And the way the people interact with the polity, and the economy, and the democratic machinery of the people as sovereign, all take place in time with reference to the order of this world.
The time of the public sphere is homogeneous, profane time. We may think of religious time or eternity as also existing, but the business of the public sphere is conducted in this world's time. And the business of the public spheres of other societies are also conducted in time - in the same time and in the same relation to time.
The modern public sphere makes the "public opinion" of the modern social imaginary essentially secular in the way it is made.