All the names written in the book of life were inscribed in eternity past. We see this in Revelation 13:8 and 17:8. 

There are two ways of translating Revelation 13:8, both of which are grammatically possible: 

 (1) “whose name has not been written before the foundation of the world in the book of life of the Lamb who was slain” (ESV); or, 

 (2) “whose name has not been written in the book of life of the Lamb who has been slain from the foundation of the world.” 

 The parallel statement in Revelation 17:8 clearly indicates that (1) is correct. 

There we read: “The beast that you saw was, and is not, and is about to rise from the bottomless pit and go to destruction. And the dwellers on earth whose names have not been written in the book of life from the foundation of the world will marvel to see the best, because it was and is not and is to come” (Rev. 17:8).

 Also, whereas it can certainly be said that the Lamb of God was “foreknown before the foundation of the world” (1 Peter 1:20) and that he was “delivered up [to die] by the predetermined plan and foreknowledge of God” (Acts 2:23), what can it possibly mean theologically to say that the Lamb of God was “slain from the foundation of the world”? The point of the text is that the people who worship the beast do so because their names have not been written in the book of life.’ That is why they are deceived by the Beast. 

 By Pastor Sam Storms