Though I believe the empire against which we struggle is Babylon, yet it is true that at this time and place (the West in the 21st century) Babylon is utilizing global consumerism as a tool of empire. Elsewhere, such as in the Islamic states, she uses other tools, older ones perhaps. But Walsh and Keesmaat are concerned with the West, so let us go forward and continue to follow their argument.
Empires seek stability by influencing every aspect of life, especially the intellectual and creative lives of its subjects. This can be seen in recent empires' concerns to control the curricula in schools, in the Hitler Youth movement, and the state-approved art of the USSR, the Third Reich and the Cultural Revolution. All these examples may seem obvious, and crude, because they are not our current culture and so can be seen by us more objectively.
If global consumerism is the economic structure of the empire, postmodernism is its intellectual structure. Postmodernism "on the street" is a deep-seated skepticism of all grand metanarratives, all grand stories or accounts that claim to organize or explain all of life. It is the doubt that truth is knowable, perhaps even the doubt whether Truth exists at all. Instead of believing or acting out of the convictions of a single metanarrative, the postmodern individual notes the existence of a plurality of differing organizing stories. He or she feels free to choose among them, even to the extent of picking them apart and selecting an account of origins from one and an account of sexuality from another. It is like mixing and matching clothing and accessories at the mall. In this way it is the perfect match to global consumerism. It is, in a sense, a "marketplace of ideas," or global consumerism of the mind. Furthermore, just as no one is expected to commit to one outfit or one laptop or one cellphone brand, so it is seen as backward and naive to commit to any particular philosophy or metanarrative combo as finally authoritative.
In all this, I think W and K are right on. Postmodernism is the official and sanctioned philosophy of the empire in these her Western provinces. A large portion of the book is devoted to exploring how the truth claims of God's kingdom can be brought forward in a manner comprehensible to a postmodern citizen of the empire. As I am not myself a postmodern man, I found this portion of the book most challenging. More on this later...