Start with the word. Resurrection was not a soft term for life after death. In first-century Judaism it meant God raising the dead bodily, all the righteous together, at the end of history. The Pharisees expected it, the Sadducees denied it flatly, and most Greeks and Romans thought a body coming back was neither possible nor desirable.
N.T. Wright's long study of the period sets that settled meaning beside what Christians were saying within a generation, and finds a set of changes too sharp to be drift.
It moved from the edge to the centre. For Jews resurrection was one belief among several and rarely the point of anything; in every early Christian document we have, it is the point.
The range collapsed to one option. Judaism held a spread of views about the state of the dead. The Christians say a single thing, and they say it about a body.
The event split in two. Nobody was expecting one man raised ahead of everybody else, in the middle of history, with the general resurrection still to come. Paul has to reach for a harvest word, firstfruits, because the category did not exist before he needed it.
And it attached itself to messiahship, where it had never been. No surviving Jewish text expects a Messiah to be executed and raised. A crucified claimant was a failed claimant; there were other claimants in that century, and that is what happened to them.
Then the week changed. Devout Jews who had kept the seventh day their whole lives began gathering on the first — Luke reports it as the habit at Troas, Paul assumes it at Corinth, Revelation already has a name for the day, and an early church manual takes the practice for granted. They also took up two rites, baptism and a shared meal, and both of them rehearse a death and a rising.
C.F.D. Moule put the historian's question about the church's own existence bluntly: “If the coming into existence of the Nazarenes, a phenomenon undeniably attested by the New Testament, rips a great hole in history, a hole of the size and shape of the Resurrection, what does the secular historian propose to stop it up with?”
The honest limit, as everywhere on this page. None of this settles what the disciples saw. A critic can grant every change listed here and answer that the cause was a set of visionary experiences that felt to those men like the new age starting, which is exactly the objection below, argued at full strength. What the changes establish is the size of the cause required: something bent a conservative tradition out of its shape, quickly, in public, and the people it happened to gave one reason for it.