This is not addressed to Christians. Christians have a part in the first resurrection, when they become immortal, and they will reign with Jesus for 1,000 years over the rest of mankind, who have a part in the second resurrection at the beginning of the 1,000 years. As Revelation 20:6 says:
(6) Blessed and holy is he who has 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 one thousand years.
so verse 8 cannot be addressed to them.
Chapter 21 is describing the events at the end of the 1,000 years reign of Christ, after the judgement of mankind. Having just described that there is a new heaven and a new earth, that all things have been made new, it says that those that overcame the trials during the 1,000 years reign shall inherit "these things" (not "all things"), i.e. the earth, the earthly kingdom, just as the overcomers during the Gospel Age inherit the spiritual kingdom.
Now that there will be no more death (verse 4), Jesus will then turn the kingdom over to God - 1 Corinthians 15:24-28 (WEB):
(24) Then the end comes, when he will deliver up the Kingdom to God, even the Father; when he will have abolished all rule and all authority and power.
(25) For he must reign until he has put all his enemies under his feet.
(26) The last enemy that will be abolished is death.
(27) For, “He put all things in subjection under his feet.” But when he says, “All things are put in subjection”, it is evident that he is excepted who subjected all things to him.
(28) When all things have been subjected to him, then the Son will also himself be subjected to him who subjected all things to him, that God may be all in all.
God will then dwell with mankind, and they will be His sons - Revelation 21:3 (WEB):
(3) I heard a loud voice out of heaven saying, “Behold, God’s dwelling is with people, and he will dwell with them, and they will be his people, and God himself will be with them as their God.
The "holy city, New Jerusalem" (verse 2) that is described as descending from heaven, is symbolic of the Government of God, comprising of Jesus and the resurrected and glorified Church, which is the tabernacle and temple of God. This will all be invisible to people living on the earth. Revelation 21:9-10 (WEB):
(9) One of the seven angels who had the seven bowls, who were loaded with the seven last plagues came, and he spoke with me, saying, “Come here. I will show you the wife, the Lamb’s bride.”
(10) He carried me away in the Spirit to a great and high mountain, and showed me the holy city, Jerusalem, coming down out of heaven from God,
Verse 8 says that those that fail to overcome will suffer the second death. Only perfected, righteous people will be allowed to live forever (there will not be people being tortured for all eternity!).