The Anti-Gnostic, a valued commenter on this blog, outlines the first approach/understanding/path in a reply to the question of when Christianity would reignite the West (bold added).
It won't. Christianity requires a Christendom with geographic territory and artifacts, which Christians blew up after two World Wars against each other. When Christians aren't protected or don't protect themselves, they go extinct like anybody else.
The religions which actually propagate themselves maintain blood-and-soil homelands or enclaves that support natalism (literal Creation, in which nominal Christians no longer participate), like Islam, Judaism and the Anabaptists.
Religion as metaphysical abstraction is just someone's, anyone's opinion. It has no tangible or tactile representation, no means for inter-generational transmission, no temporal events or life-cycle rituals. It actually eschews tangibility and tactileness as mundane and mortifying. It's just the burden of praxis with no benefit other than a narrative of an unknown, theoretical afterlife.
Bruce Charlton outlines the second approach in response to the same question. Note, that the commenters were unaware of each other’s comments when they posted their thoughts on the matter (bold added).
This is why I try, more and more, to emphasize that Christianity ought-to-be about the next world; in the sense that our understanding of this world, and what we should be doing, comes After we have decided to follow Jesus to life everlasting in Heaven (and do what is necessary to attain that).
In contrast; there is a common discourse (and I used to be deeply into this) that evaluates Christianity in terms such as its ability to save Western Civilization from itself, or improve the conditions of My life - soon.
It is natural to desire protection from suffering, yet in an entropic world leading towards death; this is ultimately futile, and therefore should Not be allowed to take primacy. ...As it so often does take primacy; in both explicitly-secular and fake-Christian discourse.
In my view, questions concerning the future of Christianity boil down to the cardinal choice between these two approaches, understandings, and paths.
The choice individual Christians make in this regard will determine the future of Christianity -- or, put another way, if Christianity has a future at all.
Note added: I believe that Christianity has a future. For me, the big question centers more on the question of what Christianity will be in the future, which of course depends on the present choices and assumptions of Christians.