Jesus told a few guys they wouldn't see death before he returns, so the theorists infer that he must have returned within ~40 years / their lifespan. I think it also correlates to the downfall of Rome and the temple destruction
Discussion
Well, he rose from the dead, that's pretty standard. I guess this is saying that that return was it? Personally I do think both the temple destruction and the fall of Rome were God's work. The temple destruction signalled the end of the warranty for whoever persisted in the false Jewish beliefs, which Jesus corrected.