How can you believe things like a man coming back from the dead, bringing a corpse back to life, walking on water, instantly healing the sick and disabled, changing the weather, ascending to heaven (did he float up into the air or what?), etc. literally happened, as historical events?
I don't. But I can understand (I think) something of how others might. After all, most of use have beliefs about the nature of reality based upon our trust in so-called experts and authority. We may even accept positions/claims that would have seemed to us to be patently absurd were it not the position of most or all specialists in relevant fields. Mathematicians, for example, inform us that there exist infinitely many different infinities. Thermodynamics relies on the fact that the probability of randomly picking any rational number from any interval on the real number line is 0, despite the fact that the rational numbers are infinitely close to one another (there are no "gaps" for any other numbers to exist within, despite the fact that physics and mathematics demands that in these non-existent gaps there must be so vastly many numbers that the set of rational numbers is negligible and can be ignored). High energy physicists, particle physics, theoretical physicists, etc., tell us that most if not all particles that make up all physical reality are virtual and that there is no real way to determine whether a fundamental process or physical system is virtual or "real" (or whether physics describes any objective reality at all).Biologists have generally shunned attempts to define their own field in such a way as to define "life" that would allow any scientific discrimination from a living system that has died vs. a non-living system, let alone what it means for a system to have been living, than to have died, than to be a living system again (physicists and biologists who attempt to define what it means to be a living system in a scientific, systematic way have tended to make clear that this demands something beyond known physical laws). Literature on fundamental physics yields what one would likely dismiss as pseudoscience were it not published in peer-reviewed journals by eminent physicists (and often in preeminent science journals) :
"We now know that the moon is demonstrably not there when nobody looks."
Mermin, N. D. (1981). Quantum mysteries for anyone.
The Journal of Philosophy,
78(7), 397-408.
"So, what is the message of the quantum?...the distinction between reality and our knowledge of reality, between reality and information, cannot be made."
Zeilinger, A. (2005). The message of the quantum.
Nature,
438(7069), 743-743.
“The only reality is mind and observations”
Henry, R. C. (2005). The mental universe.
Nature,
436(7047), 29-29.
We are told things such as that the cosmos consists of many more dimensions than could be possible (but is made possible by mathematical trickery), that split hairs split the universe, that there exist universes in which laws of physics differ and within which realms of fantasy quite literally exist, all within mainstream physics literature. Solutions to Einstein's equations allow logical paradoxes. Attempts to rid mainstream theory from appearing to suggest design result in fantastical proposals requiring mathematical theory that nobody has yet to formulate.
Schrödinger once wrote that history is the most fundamental science. I don't agree, but it is not hard to see why a physicist in his position would seek refuge in history after the entirety of the most fundamental and oldest sciences was not (as expected) finally finished but instead collapsed utterly (along with most of the mainstream conception of mathematics, formal systems, logic, etc.).
Seriously. This perplexes me. If someone was literally doing that stuff, it would be the biggest thing in the history of the world. Corpses coming back to life and walking around! But the only writings about are mythological writings from Christians, decades later at best. No one else noticed?
This perplexes you because you are biased and prejudiced by modernity. In antiquity, even the most literate scholars distrusted texts. Nobody really thought magic, miracles, etc., to be anything other than obviously real components of the natural world. The founders of history in antiquity treated myth as a legitimate historical source, described historical events in ways that included divine actions, miracles, magic, etc., and ascribed divinity to historical figures. Also, the NT largely predates the emergence of Christianity (in particular, Paul's letters date from a period in which the religious movement was a Jewish movement). It isn't that nobody else noticed, just that nobody cared until enough followers of Jesus came to be for the Jesus sect to be taken seriously.