I was asked this in an online forum, and my answer got too long to manage. Here’s my infallible prediction. How did I do?
- Orthodoxy: Little visible change, zero substantive change. Increasing numbers and cultural impact. Progress toward a single American Archdiocese, but still not there yet.
- Catholicism: Neither women priests nor married priests will happen. Increasing disaffection among liberal American Catholics leading to a significant decrease in attendance. Identification as Catholic will be increasingly cultural rather than creedal. This trend, combined with decreasing numbers of men seeking the priesthood, will force additional parish churches to close. This will be slightly offset by conversions from Protestantism, resulting in American Catholic liturgy and pastoral care becoming effectively more traditional. In northern Europe, Catholicism may fade into a cultural memory, but in North America, a leaner, more boldly traditional Catholicism will recover its equilibrium and continue to be a voice of conscience and stability.
- Reformed Christians: Continuing personality issues, but overall the hardcore Reformed will still look and act a lot like they do today, because (almost uniquely among Protestants) Reformed folks know and value their tradition. The edgy/emergey segment will contribute a few cultural differences.
- Lutherans, Episcopalians, Presbyterians, UCC: Increasing convergence so that they resemble each other almost interchangeably, while de-emphasizing troublesome doctrinal issues until their emphasis on social issues rather than personal salvation turns them into Christian-branded social service agencies. In each movement the conservative outliers will continue to peel off in schisms embodying a previous generation’s norm. Many of these etremely conservative daughter groups will identify strongly with the little-o orthodox “Great Tradition” (cf. Tom Oden)
- Anglicanism outside the US and UK: Few significant or visible changes, except increasing numbers, especially in Africa and Latin America, where Anglicanism is conservative in liturgy and ethos.
- Conservative Anglicans, Lutherans, Methodists in North America: I foresee growth and prosperity for individual parishes and dioceses, but overall a continuing fragmentation. Fifty years ago, these groups were culturally relevant and could provide nostalgia for returning Christians; now, in increasingly-unchurched America, they’re culturally unfamiliar but not yet old enough to make a virtue of ancient weirdness the way Eastern Orthodox do.
- Non-charismatic nondenominations will find their kids growing up effectively charismatic nondenoms, with more affinity for styles of contemporary music than for their parents’ doctrinal self-definition. The trend of people choosing congregations based on music and childcare will continue to grow. In-depth teaching of historical doctrine will continue to be a novelty.
- Baptist will continue to be a useless word for describing a set of beliefs or practices, as practically every form of Christian belief and worship can be found among self-described Baptists.
- Charismatic nondenominations: Same story. In the absence of doctrinal accountability, these will continue to generate new approaches and practices every decade or two.
- Old-school Pentecostals (e.g. Foursquare, Assemblies of God, Church of God in Christ, Church of God of Prophecy) in suburbs and wealthy areas will continue to develop in unpredictable ways, ensuring that every generation is nostalgic for a lost experience and baffled by what their churches have become. In the 90s and 00s, the Toronto Blessing, Brownsville Revival, Kansas City Prophets, and new prosperity teachings revisited mid-20th-century phenomena such as the Latter Rain, Manifest Sons of God, and the earlier health-and-prosperity movement springing from EW Kenyon. But the more recent iteration was characterized by a new cultural ignorance of Christian belief or history, which freed it to become crazier, faster. By contrast, change in urban, inner-city contexts and in rural areas will be minimal: Urban churches will continue to be matriarchal and rural churches patriarchal.
TL;DR: in 50 years you’ll see recognizable Orthodox, Catholics and Reformed… and a vast spectrum of Everybody Else, many of them changing in significant ways and seeing that as a virtue.