Bart Ehrman does a great review of this topic in a book and here: https://ehrmanblog.org/tag/rise-of-christianity/
“So, mr fancy pants, if Christianity is false then why is it so big? Why is it here at all?”
True - Christianity seems to have arrived in human history right when a population apex happened. Isn't this proof that God intervened at the right moment?
Well, not necessarily. Mankind is a spiritual creature, so *something* was going to persist. If it hadn't been Christianity then it would have been something else. Several of the religions around at year 0 are still here. After 2,000 years Christianity can only claim ~25% of the human population (with orders of magnitude fewer than this actually taking it seriously). That doesn't sound divine to me. Judea's back was broken by the Romans with the temple's destruction so they were looking for a reason to not need it – Christianity is it. A few hundred years later, the Roman empire imploded and the edit of Milan happened and… poof.
Furthermore, the Christian ethos was a powerful motivator for the expansion of western civilization. So, we also have a chicken-egg problem.
“But, the bible!”
Yep. Men assembled the bible from a selection of many, mostly anonymous books. They chose books that encapsulated their beliefs, so it's not a shock that it reflects a coherent position. The books that contradicted what they thought were rejected. Its existence is not a surprise. At all. It would be a surprise if they'd selected utterly contradictory books. Many of the books selected are known to be forgeries. Others have been modified over time. We don't have originals. When two people read the bible, each extracts their own meaning. In this sense, there are as many different Christian Churches as there are Christians.
“But Christianity has been miraculously preserved!”
Which one? There are thousands.
Besides, no version of Christianity now bears significant resemblance to Christianity even 500 years ago. At that point in human history deeply devoted believers were burning each other at the stake for pedantic and utterly trivial theological disagreements. Fast forward to now where “its relationship, not religion” – a subjective mess of pure emotion and hipster-ish bands in rent-a-mall spaces. It started as a cult with some good ethics, grew into a domineering monster, and has now collapsed into a self-help group. Even the most legalistic form (Catholicism) has been totally routed into just a ritualistic real-estate empire. Christianity provides hope for many and, on balance, is probably a force for good. But, it is nothing like it used to be. Christianity can better be thought of as a placeholder term with its contents meaning something different from time to time, place to place. It has evolved.
The core message is “God loved you, allowed himself to be beat up and then be dead for part of a weekend to save you from the things he will do to you if you if he hadn't done that to himself.” While confusing, it provides the proxy sacrifice that many find hopeful. The Jewish practice of scapegoating is the same thing; take the things you wish weren't so about yourself, pass them off onto another entity and start over. It reflects the deepest longing of the tragedy of the human condition… to forget what we've done wrong and to be truly forgiven. To start over.
So what happened? Why the implosion? Complicated no doubt… but I suspect it has to do with fewer and fewer answers being provided over time. We fickle humans pay attention to things we find important and fewer people are paying attention to religion. Science has given us an explanation for just about all of the physics that we interact with. We know where rain comes from. We know about lightning. We understand the atom. We can produce custom life forms. All that is left to explain is “why am I here?” This is entirely subjective… so it makes sense that the current state of Christianity only really tries to address this.
Suspiciously absent from this is a God entering the discussion to clarify things. If it is the case that a God is behind things and it started the Christian church – why stand by while it atrophies in agony begging for it to intervene? This is tragically sad to me.