The phrase "book of life" is found only eight times, in eight verses, in the Bible. These occurrences are all found in the New Testament. Only one is written by the Apostle Paul (Philippians 4:3) while the remaining seven book references are recorded by John (Revelation 3:5, 8, 17:8, 20:12, 15, 21:27 and 22:19).
According to one Biblical commentary, this phrase comes from an ancient custom of keeping genealogical records in a book (Nehemiah 7:5, 64, 12:22). We often wonder why God does things in a certain way or does not do other things. Clearly, an all-powerful and all wise deity, who knows every hair on our head (Matthew 10:30), does not need written materials to remind him of some fact.
That said, he has chosen to reveal to us a special book he has created for reasons we might not fully understand. Guided by his Holy Spirit, however, we can at least partly comprehend its purpose and the role it will play in his plan of salvation (1Corinthians 2:7).
Its Mysterious Purpose
We start to get a sense of the mysterious purpose of this volume from Paul's only reference to it. Near the close of Philippians, he exhorts two female helpers in the gospel to be of the same mind. He then requests the following.
And I ask you, my true yokefellow, to assist these women (Euodia and Syntyche), who labored with me in the gospel, and with Clement, and the rest of my fellow workers, whose names are in the book of life (Philippians 4:1 - 3, HBFV throughout).
The Biblical reference to this unique book, found in Revelation 20, is the one most familiar to people. It is the passage concerning God's judgment on the vast majority of mankind.
And I (the apostle John) saw the dead, small and great, standing before God . . . and another book was opened, which is the book of life. And the dead were judged out of the things written . . . (Revelation 20:12).
Anyone not found written within this eternal ledger will ultimately be cast into a lake of fire and perish forever (Revelation 20:15). Another place where this phrase occurs is a statement about the understanding those who are not converted will have at the end time (Revelation 17:8).
This volume appears again when John, in vision, sees the new heaven and earth created by God (Revelation 21). In this new creation, he also sees the New Jerusalem coming down to earth. Not everyone, however, will be allowed in the new city, but only those "who are written in the Lamb's book of life" (verse 27).
The Last Appearance
In this phrases' last recorded Scriptural appearance, we are warned not to add to, or subtract from, God's words or else our names will not be in this volume of life.
And if anyone takes away from the words of the book of this prophecy, God shall take away his part from the book of life, and from the holy city, and from the things that are written in this book (Revelation 22;19).
The conclusion we can draw from the above verses is that the book of life contains the names of those who will be granted eternal salvation. They are those who have repented of their sins, accepted Jesus Christ, and have God's spirit within them. They will be given the gift of being able to enter the beautiful new city of Jerusalem.