I believe this is one of the most understated roles of the church. Partly because the church has to risk its very life to do this. For me, most mega-churches ride the wave of cultural acceptability. They rarely risk unmasking the "prevailing plausibility structures" which is how Newbigin describes the principalities and powers. What most culturally driven congregations do is baptize the prevailing culture and say something like, "You just need to add Jesus to your life and it will all make sense." The key here is that culture isn't questioned, and the only thing missing is a spiritual component that will "round out" a person's life - meaning and fulfillment will follow.
The irony is it fails to ask anyone to give up anything (except for money to build the facilities that give the impression of success!) I have a friend that would like to challenge the title of a book "From Success to Significance". He would like the next book to skip the 'significance' stage and move straight from success to sacrifice. It think Jon's right. It isn't until we unmask the things that have power to shape and guide culture and us, that we will begin to demonstrate the power of the Gospel.
Many have said that our wealth masks our needs. It gives us a false sense of security. It is true that our current economic situation is unmasking the false promises of false gods.
The church needs to consider doing this intentionally, not merely accidentally. That is we need to say no to those things that give us security and value and importance that are not born in God. We also need to unmask the motives of holding power and influence in our seemingly spiritual acts. The reason my friend suggests that we need to skip the significance part of the above equation is that we can still use all our manipulative and power holding skills to move ahead a very spiritual agenda, and God can be no where in sight. The means are the end for true spiritual success. So we need to give up our own powerful means so that God's means can be seen clearly.
I did a research project some years ago. In it I asked if preaching shaped the missional practices and character of newly forming congregations. The surprise wasn't that it did, but how much it did. The most interesting part of the research to me, however, was that what 'wasn't preached' equally had an effect on the church. In those congregations where social justice and prophetic challenges were absent, so were justice practices and prophetic actions. What we don't say speaks volumes.
At this point it is Paul's admonition in 2 Corinthians that informs me. "We have this treasure in fragile clay vessels to prove that this light doesn't come from us, but comes from God."
Principalities (princely or governing powers) and powers (culture shaping energy, say like advertising or nationalism), that tell us "the way things are" can only be unmasked when we demonstrate that they are wrong. We do this in weakness and reliance. We do this in brokenness and vulnerability. We get there through tragedy and sacrifice and identifying our lives more closely with those who have not, than those who have.
Why else do you think Jesus was born in a manger in Bethlehem? Jesus unmasked the principalities and powers in his incarnation. We need to get inside Jesus' life and live there.