Jesus expected a "son of man" would come with rewards while he was on earth, to his own generation, some of whom would not taste death. He explained to his contemporary disciples how they had to prepare themselves.
Paul taught that the Coming would take place in his own lifetime - "we who are still alive".
The Revelator ends the NT with the teaching that the Coming was "soon" - not centuries later.
In each age, people have calculated that the Coming was imminent.
The lesson that history teaches is that people do not learn the lesson that history teaches.
Doug