troodon said:
In Revelation, death is thrown into a lake of fire (Revelation 20:14), destroyed, prior to the founding of the new Jerusalem. But in Isaiah, people are still dying in the new Jerusalem (Isaiah 65:20).
Am I missing something? 
Here's a pretty big piece of theology, but I'll do my best to answer:
Prior to the New Jerusalem (Rev 22), after the Second Coming, there is a 1000 Millenial Reign of Christ, after which there is a final judgement. Many variations in Christianity hold to three opinions as to how this Millenial Reign will occur, but the most literal (here I go again) is right off the text in Rev 20...
Rev 20:2 And he laid hold of the dragon, the serpent of old, who is the devil and Satan, and bound him for a thousand years;
Rev 20:3 and he threw him into the abyss, and shut
it and sealed
it over him, so that he would not deceive the nations any longer, until the thousand years were completed; after these things he must be released for a short time.
Rev 20:4 Then I saw thrones, and they sat on them, and judgment was given to them. And I
saw the souls of those who had been beheaded because of their testimony of Jesus and because of the word of God, and those who had not worshiped the beast or his image, and had not received the mark on their forehead and on their hand; and they came to life and reigned with Christ for a thousand years.
Rev 20:5 The rest of the dead did not come to life until the thousand years were completed. This is the first resurrection.
Rev 20:6 Blessed and holy is the one who has a part in the first resurrection; over these the second death has no power, but they will be priests of God and of Christ and will reign with Him for a thousand years.
Rev 20:7 When the thousand years are completed, Satan will be released from his prison,
Rev 20:8 and will come out to deceive the nations which are in the four corners of the earth, Gog and Magog, to gather them together for the war; the number of them is like the sand of the seashore.
Rev 20:9 And they came up on the broad plain of the earth and surrounded the camp of the saints and the beloved city, and fire came down from heaven and devoured them.
Rev 20:10 And the devil who deceived them was thrown into the lake of fire and brimstone, where the beast and the false prophet are also; and they will be tormented day and night forever and ever.
There are still mortals living during this time, as evidenced by the fact that they can be decieved at the end of the 1000 years (hard to imagine, but sin is still possible until the final judgement).
