John equates the second death with the Lake of Fire, the final judgment of the incorrigibly wicked, those whose names are not found in the Book of Life. While these events occur after the Millennium, the Lake of Fire is also shown to exist before the Millennium (Revelation 19:20). Whether this means the Lake of Fire exists throughout the Millennium—perhaps as a vivid reminder of God's judgment—or it is manifested only at the endpoints is not clear.
The Book of Life, mentioned twice in this passage, is first used in Exodus 32:32-33 where Moses beseeches God to forgive Israel after the Golden Calf incident: “Yet now, if You will forgive their sin—but if not, I pray, blot me out of Your book which You have written.” The Lord responded, “Whoever has sinned against Me, I will blot him out of My book.”
In Psalm 69:28, David pleads for God's help regarding his enemies: “Let them be blotted out of the book of the living, and not be written with the righteous.” He may have been referring to this same book when he wrote, “And in Your book they all were written, the days fashioned for me, when as yet there were none of them” (Psalm 139:16).
In a scene reminiscent of Revelation 20:12-14, Daniel describes the future judgment of the Beast with books being opened, and the Beast being thrown into flames (Daniel 7:10-11). In another prophecy of the same general time, Daniel 12:1-2 records:
At that time Michael shall stand up, the great prince who stands watch over the sons of your people; and there shall be a time of trouble, such as never was since there was a nation, even to that time. And at that time your people shall be delivered, every one who is found written in the book. And many of those who sleep in the dust of the earth shall awake, some to everlasting life, some to shame and everlasting contempt. (Emphasis ours throughout.)
In Philippians 4:3, Paul urges the Philippian congregation to “help these women who labored with me in the gospel, with Clement also, and the rest of my fellow workers, whose names are in the Book of Life.”
In the letter to the church at Sardis, Jesus promises that those who overcome will not have their names blotted out from the Book of Life (Revelation 3:5). Revelation 13:8 and 17:8 show that those who are not written in the Book of Life will be deceived and influenced by the end-time Beast. Being written in the Book of Life grants entrance into the New Jerusalem (Revelation 21:27), while “tak[ing] away from the words of the book of this prophecy” will result in God “tak[ing] away his part from the Book of Life, from the holy city, and from the things which are written in this book” (Revelation 22:19) Clearly, having our names in this Book makes all the difference, both in the time of the end and in our final judgment!