Preliminary Note: This essay concerns the future of Western, especially American, Christianity as opposed to non-Western Christianity which represents a different trajectory and future.
PART I: COLLAPSE
Christianity will likely collapse on many fronts in the near future. The following are ways in which this will probably transpire.
Collapse of Unifying Authority
Christianity will experience a near total collapse of any central unifying authority. The church of the future will suffer from entropy at a historically unprecedented level. This will affect doctrine and institutional hierarchy.
The church will institutionally collapse as part of the Protestant Reformation’s endgame. The forces of division unleashed during the Reformation cannot be stopped (thousands of denominations), and these forces will continue to accelerate until denominational differences are no longer discussed.  Proliferation of beliefs and groups will continue until no-one can track the divisions produced. 
Doctrinal unity will collapse. “Nothing is sacred.” Everything has and will be deconstructed to varying degrees by varying groups and individuals.
Sola scriptura will fail as an authority source because the unlimited information available on the internet renders doctrinal uniformity within a group impossible. No leader can know more than their flock, and they won’t be able to command unity or authority. A quick Google search yields five interpretations of any scripture.
The rise of the “Spirit” in the Pentecostal and Charismatic traditions will dominate much of Christianity in the near future, and it will undermine authority through the egalitarianism of spiritual gifts imparted on lay people.  More chaos and unorganized division. Competing prophesies and revelations.
Ethnic variation and conflict will tear the church apart by creating race based theological divides. Different ethnic and racial groups will split from “white Christianity” and form their own meta-narratives. This is already obvious in the ubiquitous racial segregation America experiences on Sunday morning. “Black theology” was only the beginning.
Corporate Participation Collapse
People will stop going to church or participating in corporate Christian meetings. They will also stop identifying themselves with Christian institutions or denominations. This category of divorce has been happening across American society in a number of secular situations, and it’s already affecting churches. 
Deracination will increase. The faith is primarily spread through family units and cultural structures. For the following reasons, these links will be increasingly severed.
Deterritorialization (globalization, residential instability) and increased social investment in the digital world will remove people from the traditional links that instilled the Christian worldview. One example: traditional congregations are giving way to house churches, and house churches will give way to non-attendance.
Diversity is a major accelerator of entropy. It will play an increasing role in the near-term collapse (and long-term reassertion).
With increasing diversity (of doctrinal beliefs, ethnic groups, etc.) people will be less psychologically capable of investing in community, and they will withdraw themselves to watch TV. 
The end of Christianity as an ethno-political identity, “America is a Christian nation,” will rob it of a dependable support base from which to draw the more spiritually minded into deeper commitment. Christianity won’t be inherited by each generation as a default aspect of their identity. The short-term result will be Christianity’s erasure from public consciousness.
Secular institutional suppression will play a role in stigmatizing Christianity/Christians and breaking up our institutions.
LGBT Civil Rights and other forms of moral oppression formalized by the government will be used to legally and financially suppress Christian institutions like colleges and churches (hiring regulations, etc.). Furthermore, Christianity will increasingly be seen as “criminal” and lose it’s wholesome image.
Corporations will continue to expand, and the capitalists have shown little tolerance toward Christian belief. They will use advertising, hiring standards, employee discipline, and HR brainwashing to encourage their version of “tolerance” and downplay the importance of religion and spirituality… it’s good for business if people think mostly about product consumption.
Philosophical Displacement (Liberalism)
There are deep philosophical reasons for Christianity’s continuing decline and collapse in addition to the above factors.
The doctrines of equality and liberty that emerged during the so-called Enlightenment period of thought have displaced Christian doctrine as the first principles of Western ethics. People no longer ask themselves: “Is this in accordance with God’s design?” but rather: “Does this further equality and liberty, is it ‘fair?’”
The modern Westerner assumes that because the Bible doesn’t condemn slavery, and seems to support it, it must be irrelevant to the moral essentials (equality and liberty). Many thinkers trace the decline of biblical inerrancy to the moral crusade against slavery. 
As long as philosophical liberalism remains the core of Western ethics, Christianity will remain a sideshow playing a “me too, me too” catch up game.  Christianity being attacked or critiqued by an external standard, even by Christians themselves, renders it irrelevant as an essential all-encompassing worldview.
With religion’s displacement from the philosophical core, materialism rises to inevitability. Mammon is the alternative. Money has become modern man’s real and relevant god.
The incentives to maintain a moral lifestyle for the average Christian have largely collapsed. For reasons inside and outside the church, moral commitment, especially as a young person, has been disincentivized. There’s little need to describe extra-church incentives for immorality, the following will deal with intra-church issues.
Modern churches glorify the “testimony.” The more extreme the testimony, the more inspiring the life of the Christian who lived it. Inspiring testimonies are those of disaster and redemption, and include a turning point in which the Christian repents from a life of sinful chaos and accepts the grace of God. 
While this type of testimony is inspiring because it parallels the Bible’s fall and redemption narrative, it implants within young people the idea that true Christians first experience the horrors of sin before turning to God. This inadvertently creates heroes and archetypes out of Christians who have sordid pasts. The average Christian youth comes to subconsciously believe their Christian faith is inadequate without a desperate story.
The effects of this Christian testimony culture, and the ease with which the world accommodates it, has led to moral collapse among Christians. Morality is no longer superior. Being a responsible Christian from birth to death is now boring and irrelevant to most Christians.
The “purity culture” movement’s failure represents the accelerating effect of moral collapse. An attempt to reassert sexual purity as an ideal faced deep opposition within the church itself (probably because most Christians are not virgins on their wedding nights). Interestingly, opposition has also arisen from the jaded and discouraged virgins who believed they could find a spouse who “waited,” but met the reality of moral disintegration within the ranks of their Christian peers.
Christianity’s near universal rejection of purity culture, even by the virgins who sacrificed for it, represents a devastating and probably unrecoverable failure for Christian morality. Young people have learned there’s no calculable reason to “wait.” 
Conclusion of Collapse
Western American Christians are now exiles. We will soon have no shared geographical, doctrinal, or institutional “homeland” from which to operate, and the moral quality of our members is deeply eroded.
PART II: REASSERTION
The exiles will seek their roots. Like the Jews in Babylon, we will search for something solid. The reassertion will enter our individual minds when we become conscious of our atomization, misery, and lack of identity or purpose. We will look into the historical legacy of our people to find meaning in life (we’ll realize football fandom won’t save us).
A Fresh Dark Age
Accelerating trends will destabilize our living conditions in negative ways. This will resemble the slow decay of the globalized Roman Empire (to some extent).
There will be progressively more obvious economic stagnation/decline for the average Westerner (ignoring corporate economic activity). 
The rapid increase in diversity will lead to civil war-eque conflict. Fourth generation tribal warfare resembling a low level Middle East. Different political philosophies (SJWs versus Alt-Right), ethnic groups (Latinos versus African-Americans),  religious identities (militant Islam), and others will fight for limited geography, resources, and institutional spoils in an increasingly zero-sum game.
Drug legalization and proliferation has and will increase chaos, addiction, and crime everywhere. “Deaths of despair” are already reaching crisis point in the rural areas (many, if not most, related to drug abuse of some kind).
The crisis will lead to a reassertion of reality through necessity and limitation. The result will be families, tribes, and religious identities strengthening. The evolution of a new clan(ish) structure.
Christianity will reemerge within a neo-traditional praxis comparable to the ethno-nationalist Christian identities of post-Soviet Eastern Europe (consider Poland). 
PART III: CONCLUSION/ACTION
This trajectory is 90% unstoppable (personal estimate). Nothing can halt or reverse the majority of these trends. They must play themselves out and arrive at the logical conclusion.
Should we give up and simply watch this catastrophe unfold? No.
We need to “ride the tiger,” and not exit into a total reassertion mindset too quickly. We can’t “LARP” the reassertion into existence because if we’re too far ahead of the curve we won’t get much done in contemporary society.
The proliferation of house churches, for example, represents a deterioration in the Christian landscape, but it’s also healthy in that it delays collapse and keeps Christianity within people’s consciousness until entropy climaxes. In other words, house churches are a necessary part of the decline, and should be encouraged as a way of avoiding a too rapid onset of peak atomization within Christianity (sometimes delaying a possible inevitability is solid strategy).
At this point in the trajectory, we’re accelerating towards near total collapse, and some of the more foresighted parts of society are already entering the beginnings of a conscious attempt at reassertion.
Fighting, “doing something about it,” can take several forms: (1) laying the groundwork for reassertion in the form of networking and theological/intellectual work; (2) accelerating the current order’s collapse by withdrawing investments and discrediting the contemporary system of thought/action/institutions (“Cathedral”);  (3) finally, fighting on a personal level to escape materialist philosophy and the liberal ethical system, and reasserting within ourselves a purified Christian worldview. 
 I write this as a low-church Protestant. I’m not condemning Protestantism, I’m just pointing out its results.
 Examples: “emergent Christianity,” rise of house churches, etc.
 Pentecostal/Charismatic Christianity is by far the fastest growing branch of our religion, and it’s projected to overtake Protestantism in the near future (if it hasn’t already).
 Examples: declining participation in politics, civic clubs (key club, etc.), bowling clubs, church attendance, and more.
 See Robert Putnam’s ‘Bowling Alone’ and other studies on modern Western social capital collapse, decreasing civic participation, and falling social trust.
 See the last chapters of Phyllis Tickle’s ‘Emergence Christianity.’
 I’m discussing 18th century liberalism, not the conservative slur word.
 I’ve experienced this within my own churches. The most inspiring testimony demands the highest spiritual authority. Solid Christian “lifers” are pushed aside in favor of the reformed drug-addicts, loose woman, and gang bangers. The new Christian archetypes. If you oppose this paradigm you’re the prodigal son’s older brother.
 Two Points: This failure is “unrecoverable” until the reassertion. Modern youth (Generation Z, for example) are having less sex, but not on moral grounds (encouraging?).
 It's already happening: job losses, population increase from low skilled immigration, wage stagnation, etc. American wages have been stagnantsince the 1970s.
 Read Jared Taylor on interracial Los Angeles turf wars in his ‘White Identity.’
 This is a limited comparison considering widespread diversity in places like America (and increasingly Western Europe). The Eastern European nations were able to reassert Christian identity within an ethno-state because they retained relatively homogeneous populations throughout the communist period.
 I hate the term “Cathedral” as an expression for the liberal order because architectural cathedrals are beautiful representations of orthodox Christianity.
 Perhaps this worldview could be described as “neo-barbarian” in its confidence.