In history, it seems that there have been movements to bring religion back to its origins... especially when there is written record, like the Bible, or Koran.
As a religion progresses through time, there is less direct connection with the 'written word'. Someone claims to 'have it' so people follow that person. This is where personal feelings are introduced, and rather than seeking the source, the 'second generation' incorporates the teachings of their [person in touch with God], making an internal 'copy' of the religion.
How many generations (copies of copies) can there be? The thought occurs that someone must have become aware of this degradation, and decided to seek the Source. Thus a revival.