Just about everything we know about the Book of Life is from Revelation (cf. Rev 3:5; 13:8; 17:8; 20:12, 15; 21:27; the only other reference is in Phil. 4:3).
And here’s what we know.
It is a book about life (how’s that for insightful?). It’s not that the book itself imparts life, but it is a record of those who have been granted eternal life. All whose names are in that book have life. No one has life whose name is not in that book.
Once someone’s name is in the book, it cannot be erased and removed (Rev. 3:5). A name that appears on those pages appears there irrevocably. Eternal life is just that. It is eternal life, not “life for a little while.”
The names in the book are placed there not at the moment one places faith in Christ; they were written into the book before the foundation of the earth (Rev. 13:8; 17:8). A friend once applied for membership to our church and when asked, “when were you saved?” he replied, “I suppose you could say I was saved before the foundation of the earth!” Now his salvation was not enacted at that point, but with his name being placed there by God, he (along with everyone else who is in the book), his salvation was guaranteed.
That also affirms that the believer does nothing to bring about his own salvation. Eternal life and fellowship with God is a gift of His grace through the work of Christ alone. We do not place our name in the book. The book belongs to the Lamb (Rev. 3:5; 13:8; 21:27). He writes the names in the book. He owns the book. He keeps the book. And He keeps all those whose names are in the book.
But perhaps the most striking aspect of this book is what is said of it in Revelation 13:
“All who dwell on the earth will worship him [the beast], everyone whose name has not been written from the foundation of the world in the book of life of the Lamb who has been slain” (v. 8).
From the foundation of the earth, this book has belonged to the slain Lamb. That means that before time began and creation occurred, God’s plan was for the Son of God to be slain for the redemption of a sinful people who had yet to be created. The cross was in the eternal plan of God:
This means that before the world was created there was a book called the “book of life of the Lamb who was slain.” The Lamb is Jesus Christ crucified. The book is the book of Jesus Christ crucified. Therefore, before God made the world he had in view Jesus Christ slain, and he had in view a people purchased by his blood written in the book. Therefore, the suffering of Jesus was not an afterthought, as though the work of creation did not go the way God planned. Before the foundation of the world God had a book called “the book of life of the Lamb who was slain.” The slaying of the Lamb was in view before the work of creation began. [John Piper]
This is what the Father has done in love for the Son. This is what the Son has done in His love for the Father. And this is how the Triune God has enfolded us into their love. Don’t think of it just as a book — this book owned by the Lamb is a most precious and gracious book!