The demise of "Christendom" affects both the Church's theological dialogue and missional strategy.
Michael Frost and Alan Hirsch give the historical explanation for what it is:
Christendom is the name given to the sacral culture that has dominated European society from around the eleventh century until the end of the twentieth. Its sources go to a time when Constantine came to the throne of the Roman Empire and granted Christians complete freedom of worship and even favored Christianity, thereby undermining all other religions in the empire. . .
Taken as a sociopolitical reality, Christendom has been in decline for the last 250 years, so much so that contemporary Western culture has been called by many historians (secular and Christian) as the post-Christendom culture (The Shaping of Things To Come, [Peabody, MA: Hendrickson, 2003], 8-9).
Timothy Tennent calls it a mind-set of which systematic theologies must escape:
The basic problem is that Western systematic theologies are still written with a Christendom mind-set, assuming the absence of rival theistic claims as well as rival sacred texts. They tend to be overly preoccupied with philosophical objections to the Christian message, rather than with religious objections based on sacred texts or major social traditions that contradict the claims of Scripture (Theology in the Context of World Christianity, [Grand Rapids: Zondervan, 2007], 257).
Frost and Hirsch draw out the missiological implications due to its failure:
We must admit that Christendom, particularly its ecclesiological and its missiological manifestations, amounts to something of a failed experiment. . . . Christendom is not the biblical mode of the church. It was/is merely one way in which the church has conceived of itself.
In enshrining it as the sole form of the church, we have made it into an idol that has captivated our imaginations and enslaved us to a historical-cultural expression of the church. We have not answered the challenges of our time precisely because we refuse to let go of the idol. This must change!
The answer to the problem of mission in the West requires something far more radical than reworking a dated and untenable model. It will require that we adopt something that looks far more like the early church in terms of its conception of the church (ecclesiology) and its core task in the world (missiology) (paragraphing mine, 15).