As to the degree that these are falling apart is debatable. They certainly don't have the strong central hierarchy and universalism that Catholic and Protestant sects have, but they seem to endure.

Without Paul, Christianity reverts to being a variety of Judaism whose leader from the hinterlands got it right about what really mattered in life, as had his predecessors [0]. But he fatally misjudged the big city's religious oligarchs — vassals to their ruthless Roman occupiers — when he relentlessly attacked them and their cozy little setup; at their behest, he was executed by the Roman overlords.

Some [1] of the leader's later followers — his posse, if you will — imagined they'd seen him. But the leader's wealthy and/or well-connected followers are strangely absent from the narrative. Perhaps they had more information about what had really happened [2].

The early postmortem appearance tales eventually mutated into a legend of a warrior-king, raised from the dead — who would return Real Soon Now, to usher in God's reign and establish Israel's rightful place in Creation [3].

Over decades, the tales percolated into Mediterranean Graeco-Roman culture — eventually mutating further still into a tale of a divine being [4] (perhaps hybridized with that culture's myths?).

Some self-cites:

[0] https://www.questioningchristian.org/2006/06/metanarratives_...

[1] https://www.questioningchristian.org/2004/10/troubling_incon...

[2] https://www.questioningchristian.org/2005/10/the_empty_tomb_...

[3] https://www.questioningchristian.org/2006/04/what_did_messia...

[4] https://www.questioningchristian.org/2005/11/jesus_is_lord_d...