"If anyone's name was not found written in the book of life, he was thrown into the lake of fire," says Revelation 20:15. In this context, the Book of Life refers to the list of names of those who will spend eternity in heaven with God. It is the list of those who have been saved. Revelation 3:5, 20:12, and Philippians 4:3 all mention the Book of Life. Because it contains the names of those who have been redeemed by the blood of the Lord Jesus, this book is also known as the Lamb's Book of Life (Revelation 13:8; 21:27).
How do you know if your name is in the Book of Life? Make certain that you are saved. Repent of your sins and place your faith in the Lord Jesus Christ as your Savior (Philippians 4:3; Revelation 3:5). Once your name is inscribed in the Book of Life, it cannot be removed (Revelation 3:5; Romans 8:37-39). No true believer should have any doubts about his eternal security in Christ (John 10:28-30).
Unbelievers will face judgement at the Great White Throne Judgment, as described in Revelation 20:11-15. That passage makes it clear that no one's name is in the Book of Life at the time of the judgement (Revelation 20:12-14). The ungodly's fate is sealed; their names are not written in the Book of Life, and their punishment is certain.
Some people cite Revelation 3:5 as "proof" that a person's salvation can be lost. However, the Lord's promise in Revelation 3:5 is unequivocal: "He who overcomes... I will not blot out his name from the Book of Life." An overcomer is someone who has triumphed over the world's temptations, trials, and evils—in other words, someone who has been redeemed. The saved are recorded in God's book and have the assurance of eternal security.
Psalm 69:28 also causes some consternation: "Let them [David's enemies] be blotted out of the book of the living." This "living book" should not be confused with the Lamb's Book of Life. David is referring to physical life on Earth, not eternal life in heaven. The "book" mentioned in Exodus 32:32-33 is the same.
God keeps meticulous records. He knows His own, and He has written their names in His book forever.