Istanbul is the city of Constantine (Constantinopel), the Roman emperor that - after three centuries of persecution of the church - officially recognized Christianity, launching an era in which church and empire (political power) became so interrelated, that it altered the fundamental mode (or DNA) of church. Up to this day we face the disastrous consequences of Constantinian thinking, and the call to conformism that comes with it.
As Alan Hirsch and Michael Frost point out in 'The Shaping of Things to Come', this mindset is marked by an attractional ecclesiology (come-to-us mentality, church buildings and services becoming the primary place to encounter Jesus), a dualistic spirituality (a division of the sacred and the profane, leading to an inability to effectively bring God's Kingdom into the culture) and a hierarchical mode of leadership (clergy-laity divide, church leaders primarily being pastors and teachers).
When we talk about reformation, we call for an intentional break with this Constantinian mindset in order to see the church become what Jesus - the real Founder - intended it to be: incarnational (bringing Jesus where the people are, forming community in all kinds of places), messianic (seeing the world and God's place in it as more holistic and integrated), and apostolic (five-fold ministry teamwork serving the church's mission).
When the Ottoman army of Sultan Mehmet II conquered Constantinople in 1453, the Aya Sofia, the symbol of Eastern Christianity was turned into a mosque. I'm getting more and more convinced that if we continue drinking out of Constantine's cup, and fail to turn the church back into a vital missional movement, Islam might eventually take over Europe, and the church will be forced underground.
If you're interested to explore this topic more in-depth, then check out Wiki's entries on Constantine, and the (first) Council of Nicea, as well as the history of Constantinople before and after 1453.

I am currently reading "The Shape Of Things To Come" and have been fascinated by the atractional vs incarnational approach described for the church. Very helpful concept to help us all in getting back to a "go" mentality vs a "come" one.
Your observation that Islam might take over if we don't stop drinking from Constantine's cup is a sobering thought to begin speaking up more about this situation!
Posted by: GuyMuse | June 01, 2006 at 15:30
Hi Marc,
I agree very much with this post. I will link to this post at my own weblog. Yet there are few more things to say about it, I believe:
1) From a contextualization perspective it was OK for christianity to take up Roman culture and forms and Latin language. It did so with the Greek in the Eastern part and with the Persian in the further East. The trouble is the unwillingness of the church to continue adapting to new cultural forms but to declare the Greek/Roman heritage as absolute (=same problem as at Acts 15). This lack of cultural translation was one of the major causes underlying the Protestant Reformation. It is still a huge problem in missions today.
See this post: http://www.rijneveld.eu/2006/04/vertalen-van-de-boodschap.html
2) It was former, converted Vikings who were leaders of the Crusade that for the first time in history were able to defeat Constantinople from their fellow Christians. This paved the way for the subsequent defeat by the Ottoman sultan in 1453 for it never regained its former strength.
Posted by: Wouter Rijneveld | June 01, 2006 at 21:03
Nice outline. Sorry for the Vikings.
Posted by: Are Karlsen | June 02, 2006 at 02:12