They have come to believe in it through faith. Most just believe in the cause without ever knowing why or knowing what the original inspiration was all about. Jesus, on the other hand, was killed for something He DID come up with. No sane person is willing to die for a lie they made up. This defies all logic as was discussed above earlier.
People are willing to die, however, for something they believe is genuinely true.
With Jesus, if He was lying, He would not believe it true and thus not be willing to die. Most zealots have more than their share of lunacy. BTW, if Jesus, as you said, was killed for something he DID come up with, that means he invented his purpose and message during his life and was neither born to it nor sent from God with it. I could agree with that. This brings up an interesting question.