In responding to Milbank’s analysis of Augustine on the secular, RA Markus ( Christianity And the Secular (Blessed Pope John XXIII Lecture Series in Theology and Culture) 
Markus thinks this formulation entangles two issues that need to be disentangled.
First, there is the question of whether an individual needs to refer all his actions in the public sphere to the ultimate end of worshiping and honoring God. According to Markus, Augustine’s answer would be Yes, but points out that “there is nothing exceptional about a public sphere in this respect. Acting politically is, like acting in any other sphere, never morally indifferent.” If a political act is not sinful, it is because it depends on grace.
But there is also the separate question of whether a neutral public sphere exists at all . Markus thinks that Augustine believed there was such a neutral public sphere, though “there is no morally indifferent action within it.” This conclusion rests in part on a prior conclusion about how Augustine conceived the public sphere itself. For Augustine, it was not “the sum of a multiplicity or a fabric woven of many actions by many people.” Augustine thought of the public sphere “in much less personal terms” as a matrix of “practices, customs, institutions” that are “the cumulative effect of long sequences of human action, shaped by collective behavior over many generations, routinised or institutionalised over time.” These habits function like a language, shaping and conditioning behavior but not determining the behavior itself any more “than a language determines what we say in it.” Just as Christians can make use of secular learning, “members of the two Cities make use of the same finite goods, although for different ends, with ‘a different faith, a different hope, a different love.’”
I won’t quarrel with Markus in his reading of Augustine. He may well be right. But it is less clear that Augustine (or Markus’s Augustine) is right. Of course, there are many social customs and habits shared by believer and unbeliever, where the only difference is in the faith, hope, and love by which use is made of these customs. Christians in Europe use forks; in Asia both use chopsticks.
But surely, some practices, customs, and institutions, representing the cumulative collective behavior over generations so as to become habitual are evil. Israel institutionalized calf worship from the time of Jeroboam, and Baal worship from the time of Ahab, but the prophets didn’t simply make right use of the existing customs and institutions. They attacked the existing customs and institutions themselves.
It is illuminating to think of public life as a language used by both believers and unbelievers to say their very different things. But that is too simple an analogy to capture the complexities of shared life, and that means it is far too simple to claim that the public sphere is a “neutral space.”