It is linked to Christ's reign. You see, by Christ's death and resurrection he accomplished salvation for his people, once and for all. It was finished. He cried that from the cross (John 19:30). And so we live in the Now, when that reality is realized for Christians as Christ reigns in and through us. But that reality is not fully realized until Christ returns and ushers in the New Heavens and the New Earth. That is the Not Yet. See how Michael Horton explains it in his book, Calvin on the Christian Life:
Right now ... Christ's supreme reign is not visible in the daily news; we live by promise, on the basis of already-accomplished events. We live in an intermission. [...] We live in an ambiguous tension between the "already" [or the Now] and the "not yet."
We are in the intermission of a great play. The atonement was the opening act. The return of Christ closes the curtain. So right now the Christian is living based upon the work of Christ on the cross. But they're living for the return of Christ and the restoration of all things. We live in the Now, but we long for the Not Yet.
We live in an "ambiguous tension," says Michael Horton. It's a peculiar age, living based on the promises of the past and waiting with eager anticipation for the future restoration. But it's a beautiful age, as we cling to the Word of God and use the fading time to proclaim and live out the gospel.
This is not a docile intermission, but one charged with intentional living in the Now and looking forward to the Not Yet.
Image Credit: http://www.creationswap.com/artwork/17/8/24/23849/17824_23849_5.jpg