The second death is mentioned on multiple occasions in the book of Revelation and is synonymous with the lake of fire. It is a “death” in that it is a separation from God, the Giver of life. It is called the “second” one because it follows physical death.
Revelation 21:8 explains the second death in the most detail: “The cowardly, the unbelieving, the vile, the murderers, the sexually immoral, those who practice magic arts, the idolaters and all liars – their place will be in the fiery lake of burning sulfur. This is the second death.”
Three other places in Revelation also mention the second death. The first is Revelation 2:11: “He who has an ear, let him hear what the Spirit says to the churches. He who overcomes will not be hurt at all by the second death.” In this verse, Jesus promises that believers (“overcomers”; see 1 John 5:4) will not experience the lake of fire. The second death is exclusively for those who have rejected Christ. It is not a place believers in Christ should fear.
Revelation 20:6 speaks of the second death in relation to a future period called the Millennium: “Blessed and holy are those who have part in the first resurrection. The second death has no power over them, but they will be priests of God and of Christ and will reign with him for a thousand years.” This verse notes three important facts. First, those who die for their faith in Jesus during the Tribulation will later be resurrected to enter the Millennium and live with Him. Second, these martyrs will escape the lake of fire or second death. Third, they will reign with Christ.
The second death is also mentioned in Revelation 20:14-15: “Then death and Hades were thrown into the lake of fire. The lake of fire is the second death. If anyone’s name was not found written in the book of life, he was thrown into the lake of fire.” At the end of time, even death and the grave (Hades) will be thrown into the lake of fire. In addition, every person not included in the book of life will be thrown into the lake of fire. This condition will be final; the destination is permanent.
In summary, the second death is a reference to the lake of fire where those who are separated from God by their sin will dwell for eternity. This judgment was recorded in Scripture as a warning to unbelievers to seek the salvation that Jesus Christ provides. The coming judgment should also challenge believers to share their faith. There is a vast difference between the final destination of those who know Christ and those who do not.