When Jesus was crucified, his followers were discouraged and depressed. So they dispersed. The « Jesus movement » was all but stopped in its tracks.

Then, after a short period of time, we see them abandoning their occupations, regathering, and committing themselves to spreading a very specific message — that Jesus Christ was the Messiah of God who died on a cross, returned to life, and was seen alive by them.

And they were willing to spend the rest of their lives proclaiming this, without any payoff from a human point of view.

They faced a life of hardship. They often went without food, slept exposed to the elements, were ridiculed, beaten, imprisoned. And finally, most of them were executed in torturous ways.

For what ? For good intentions ?

No, because they were convinced beyond a shadow of a doubt that they had seen Jesus Christ alive from the dead. 

They knew, without any shadow of doubt, that He really was The One and only Son of God, who proved it by rising from the dead.