For Christianity is an idea, and as such it is indestructible and eternal, as all ideas are. What then is this idea?
One of the courses I'd love to put together and lead one day is a seminar on the idea of the essence of Christianity. The mid-19th to early-20th century Das Wesen des Christentums genre hasn't been in fashion for a while, and the ideas of this essentialism probably aren't especially useful or interesting to contemporary theological work, but as a chapter of intellectual history it's a fascinating topic of consideration and instructive if not directly applicable. Heine offers a striking precursor here to the idea of a Christian "interior essence", and examples of such precursors certainly present themselves before him. As a decidedly German motif, one could even draw its pedigree back to something like Luther's identification of the doctrine of justification as "the head and the cornerstone" of the faith (although I imagine this would be pretty speculative and require a good bit of qualification to connect it to post-Enlightenment attempts to identify the essence of the faith on an ideational level). Setting aside the content and looking at the form of the Wesen tradition, the opposition of history to eternal truths is bread and butter for modern German thought and readily suggests Lessing along with a number of others.
A consideration of this essentialist tradition would also benefit from further discussion of its demise and present alternatives. An emphasis on creed, canon, and structure of the sort that Jaroslav Pelikan pushed is probably a strong contender for the most prominent current heir amongst intellectual traditions that want to say something worthwhile about "Christianity" as a whole. The Nicene Christianity volume of essays is a good recent example of this school of thought as it might synthesize the Christian faith, although one of the most interesting characteristics of a structural focus (rather than a doctrinal or moral focus) is how diverse the outcome will be... Nicene Christianity is only one of many ways to invest canonical rubrics with religious content. On the level of popular discourse, Mere Christianity comes most obviously to mind as a 20th century version of the Das Wesen des Christentums literature.