All mankind are sinners at conception. Psalm 51:5 (CSB), “Indeed, I was guilty when I was born; I was sinful when my mother conceived me.” Adamic sin and its penalty, death, are inherited. Romans 5:12 (CSB), “Therefore, just as sin entered the world through one man, and death through sin, in this way death spread to all people, because all sinned.” Thus, we are all dead.
Jesus amplified on the thought when He said, “Let the dead bury the dead.” (Matthew 8:22.) A young man, who wanted to follow Jesus, asked if he could first bury his family. To his request, Jesus explained that the dead (people who were still living and not yet lifeless) should bury the completely dead relatives. From God’s perspective, all sentenced to death are surely dead. No one condemned to death could have life.
The first time the Book of Life is mentioned in the Bible is in Philippians 4:3 (NKJV). “And I urge you also, true companion, help these women who labored with me in the gospel, with Clement also, and the rest of my fellow workers, whose names are in the Book of Life.”
After Jesus ransomed Adam (and all his children) through His death on the cross, the invitation to everlasting life was opened. 2 Timothy 1:9-10 (CSB), “He has saved us and called us with a holy calling,…made evident through the appearing of our Savior Christ Jesus, who has abolished death and has brought life and immortality to light through the gospel.”
Presently, the world of mankind does not respond to the holy calling. They will remain under the curse until Jesus’s blood is applied to them in the resurrection. However, some do accept Jesus as their Savior and consecrate their lives to obey God’s will. These blessed ones are forgiven. The Holy Spirit begets them to new spiritual lives. At that point, the person is no longer considered dead by God. Instead, his name is written in the Book of Life.
The rest of mankind will be resurrected during Christ’s reign on earth. Then they will receive forgiveness with the opportunity to gain perfect, everlasting human life. Then their names will be in the Book of Life. (See Jeremiah 31:33 and Isaiah 35.)
Will a person’s name stay in the Book of Life? Not necessarily. If a person hardens his heart, denies the precious blood of Jesus, and goes into second death, that person’s name is blotted out of the Book of Life. He will never exist anywhere again.
Revelation 3:5 (NKJV), “He who overcomes shall be clothed in white garments, and I will not blot out his name from the Book of Life; but I will confess his name before My Father and before His angels.”