It's a question of history. To oversimplify:
After the initial era of the apostles, the church settled into a basic pattern. Churches met in homes, and a "congregation" consisted of a dozen or a couple dozen people who met together each week. The fact that their religion was officially illegal (though public opinion varied from support to persecution and back) kept them growing closer together and supporting each other. Their fellowships were intimate groups of people who shared life together, caring for each other's needs, eating together, and loving each other. This pattern of care spilled over into the larger community, and these "Christians" (an insult thrown at them from the wider community who mocked them for following the crucified "Christos") soon got a reputation for tending the sick, helping the poor, and taking in the outcasts. So their fellowships grew, and soon larger cities would have a network of congregations meeting in homes scattered throughout the city. Sometimes these networks had a leader, an "overseer" (episkopos in Greek) who watched over a group of several house-congregations.
These tiny knots of Christians met, worshipped, and lived under the radar of the wider society for the most part; yet in other ways they were very public. They were not afraid to speak to others about their faith in Jesus Christ, except during times of active persecution. They actively worked to provide means to care for the sick and the outcasts. From time to time some of their leaders wrote and spoke boldly, either appealing to the wider population to consider Christianity, or to defend Christian faith against unreasonable attacks, or to rebuke others who claimed the name of Jesus but distorted his teachings.
All of this changed about 280 years after Jesus' death and resurrection, when Constantine became emperor of the Roman Empire. Up until Constantine's reign, various emperors had tolerated or persecuted Christianity, but it was always officially an illegal sect. (The Romans distinguished between a religion, which had time and tradition behind it, and a sect, which they considered simply a deviant religious movement. They classified Christianity under the heading of "sect".) Through a complex series of events still much debated by historians, Constantine became a Christian and declared Christianity a legitimate religion in 313 AD with his "edict of toleration." A short time later, in 380 AD, Emperor Theodosius declared Christianity not only a legitimate religion, but the official religion of the Roman Empire.
Now the church faced both a great opportunity and a great problem. Until now, persecution and public scorn had kept complacency in the church to a minimum. Those who converted to Christianity knew they were stepping outside the accepted boundaries of Roman society; conversion was a counter-cultural move. After Constantine and Theodosius, however, the church had Rome's carte blanche to proselytize, evangelize, missionize, preach and invite the public to consider the claims of Jesus. The sheer number of Christians grew by leaps and bounds, but the level of commitment of the average Christian dropped dramatically. Instead of being rooted in tight, highly committed fellowships, the church existed in giant congregations. Somewhere along the way, someone got the idea that one professionally trained leader could manage a larger congregation, and soon one shepherd was expected to manage and lead a congregation of hundreds of people. Instead of meeting in homes, churches built separate buildings and created an administrative structure modeled after the Roman government itself, complete with an emperor (pope), prefects (cardinals), and governors (bishops) to oversee the priesthood. Church became closely allied with government, and church leaders got concerned with public policy more than with individual piety and community life. In short, the cost of following Jesus had just been greatly reduced.
Now the average Christian was asking new questions. Instead of asking, "What is this life of following Jesus like?" converts began to ask, "How much do I have to do to go to heaven?" Instead of asking, "What do my brothers and sisters in the church need?" the average Christian began to ask, "What time is church?" and "Is the preacher any good?" Certainly there had been complacent believers over the past 300 years, but suddenly these questions became the norm rather than the exception.
This alliance between the larger society and the church is a state of being we call "Christendom" -- the kingdom of Christ allied to the kingdom of this world. So throughout Europe over most of the last thousand years, to be part of society meant to be baptized; to be a citizen meant to be part of the church. The two were one and the same. So when Martin Luther's followers were ejected from the church of Rome in the 1500's and they needed some kind of oversight for their churches since they no longer had a network of bishops, Luther turned to the secular princes and gave them the (what he thought was temporary) role of leading the church. Thus developed the state churches, where each nation had an official church, and the alliance between the kingdom of Christ and the kingdom of the world reached a new level.
All this changed in 1789, at least in terms of the West. (Missionaries in many other parts of the world had been living at the fringes of non-Christian societies for years, of course, but they were most often sent from nations that considered themselves part of Christendom, and part of what they hoped to bring to these "heathen" societies was a version of Jesus ruling over society.) In 1789, the fledgling United States of America ratified its new Constitution that included a Bill of Rights which specified a division between state and church. Suddenly Christendom was officially broken, though the consequences of this breakage have taken more than 200 years to take effect. In reality, it is only since the 1950's that this separation has become visible to the naked eye, as American culture has drifted farther and farther from the church. Church leaders are no longer assumed to have standing in the secular marketplace, and Jesus-followers begin to find themselves scorned for their faith. In Europe, where the state-church system held sway for so many years, the decay and disintegration of Christendom has been more rapid and more thorough. The pinnacle of complacent Christianity may have been expressed by the Archbishop of Sweden while that country debated about voting to end the state church system (which they did) in the 1990's. When asked about the vote, the Archbishop said, "The church in Sweden is much like the Post Office. No one thinks very much about it, but where would you be without it?"
So here we are in 2010, living in the tumbledown ruins of Christendom. In many ways it's like we're marching backward in time past Constantine, back into a post-apostolic time when the church existed at the margins of society, without political power, without automatic authority, but with growing commitment and with great opportunity to do good and reach people who are outcasts, disillusioned, and disenfranchised. Question for us is, what will we do with these opportunities? Should we try to retake society, re-establish Christendom? Or should we seize the chance to see commitment among Jesus-followers increase and seek to form knots of Christians sharing life together, meeting in homes, caring for the needy in their communities?