So you are just like all those naysayers who wish to have God give them EXPLICIT statements otherwise you will not believe anything. Well God did not give us a textbook of Systematic Theology, so you should disabuse yourself of that notion.Post the passage(s) that I must have overlooked that teach the name of every human that shall ever live is written in the Book of Life.
But God gives us many implicit statements which must be understood by the enlightening of the Holy Spirit. So let's take one passage (the words of Christ) and look at it in the light of John 3:16,17: I am the living bread which came down from heaven: if any man eat of this bread, he shall live for ever: and the bread that I will give is my flesh, which I will give for the life of the world. (John 6:51)
What is implied in this verse? Can you see it? Since all human beings have physical life, it should be obvious that this "life" of which Christ speaks is eternal life. "The world" is the world of humanity, so when Christ spoke of His crucifixion ("my flesh") as that which would give eternal life to the world, He meant for every human being to have eternal life. Which means all human beings are already in the Book of Life.
But as we know it only by believing on the Lord Jesus Christ that God gives anyone the gift of eternal life (John 3:16,17 which says that "the world might be saved"). So in fact the whole world of humanity is in the Book of Life, and then individuals are blotted out of that book when they fail to obey the Gospel.
Also, every Jew believed that he or she could have eternal life. And that is why the Lord said this to the Jews: Search the scriptures; for in them ye think ye have eternal life: and they are they which testify of me. (John 5:39) And we know that it would only be by looking to Christ Himself that they would receive eternal life. But the underlying assumption was that all could have eternal life, and that is what the Book of Life is all about.