What is the relationship between the millennial Kingdom and the eternal state?
The millennial Kingdom is the beginning of eternity and an introduction to the eternal state. Although it is stipulated as a thousand years, it will be interrupted by a new heaven and new earth. Eternity will be based on largely the same circumstances as the thousand-year Kingdom, and the righteous enter eternal life.
Les explains that the Kingdom, even though stipulated as a thousand years, is still the beginning of eternity. He notes that many theologians debate what takes place in the Kingdom age versus what takes place in eternity, but he oversimplifies by saying that all the situation concerning the Kingdom is just an introduction to the eternal. It will be interrupted, and there will be a new heaven and a new earth, but eternity will be based on pretty much the same set of circumstances as the thousand years. Jesus tells those going into the Kingdom that they are going into eternal life, and indeed they are. The rejectors go away into everlasting punishment, while the righteous go into life eternal.