The Millennium, part 2 (with Q&A at 31:29) A, Pre, or Post?

Will there be a millennium? Are we already enjoying it? Must we bring it in ourselves? For the first three centuries of the Christian church, the belief was that Jesus, at the end of the age, would return to the earth to establish a literal earthly kingdom that lasts for a period of one thousand years. Why did the original literal interpretation change? Are there any possible circumstances that might have influenced the Protestant Reformers to reject it? Did John Nelson Darby invent premillennialism? Which view is more consistent with the doctrines of salvation by grace through faith and Divine sovereignty? How should Revelation chapter 20 be interpreted?