Hey Mr. Jeffrey,
How can we know we're in the Book of Life? What if I commit a sin (drive over the speed limit, think an evil thought, etc.)? Is my name erased from the Book of Life? How does one get into and stay in the Book of Life?
"But you have come to Mount Zion and to the city of the living God, the heavenly Jerusalem, and to myriads of angels, to the general assembly and church of the firstborn who are enrolled in heaven, and to God, the Judge of all, and to the spirits of the righteous made perfect, and to Jesus, the mediator of a new covenant, and to the sprinkled blood, which speaks better than the blood of Abel" (Hebrews 12:22-24).
When a person is saved and is added to the church at his baptism(Acts 2:38-39,46-47), his name is restored to the Book of Life. This is why Paul could talk about his fellow laborers being in the Book of Life. "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" (Philippians 4:3). It wasn't a guess, but knowledge of their committed obedience to God (I John 2:3-6) and it is shown by the fruit of their efforts (Matthew 7:16-20).
When Moses came down from the mountain with the stone tablets, he found the people committing idolatry and other sins. He was so upset, he broke the stone tablets and punished the people. Still, he approached God to ask Him to forgive them, but if He would not, Moses asked that he not be saved either. "On the next day Moses said to the people, "You yourselves have committed a great sin; and now I am going up to the LORD, perhaps I can make atonement for your sin." Then Moses returned to the LORD, and said, "Alas, this people has committed a great sin, and they have made a god of gold for themselves. But now, if You will, forgive their sin--and if not, please blot me out from Your book which You have written!"" (Exodus 32:30-32). God told Moses, "no." "The LORD said to Moses, "Whoever has sinned against Me, I will blot him out of My book"" (Exodus 32:33). No one directly chooses whether he is in God's book or not. It is God's decision, and that decision is based on whether he was in sin or not. Thus, we also learn that entries into the Book of Life are not permanent.
The church in Sardis received a warning: "To the angel of the church in Sardis write: He who has the seven Spirits of God and the seven stars, says this: 'I know your deeds, that you have a name that you are alive, but you are dead. Wake up, and strengthen the things that remain, which were about to die; for I have not found your deeds completed in the sight of My God. So remember what you have received and heard; and keep it, and repent. Therefore if you do not wake up, I will come like a thief, and you will not know at what hour I will come to you. But you have a few people in Sardis who have not soiled their garments; and they will walk with Me in white, for they are worthy. He who overcomes will thus be clothed in white garments; and I will not erase his name from the book of life, and I will confess his name before My Father and before His angels'" (Revelation 3:1-5). Some were not living as they ought. If they did not repent of their sins, their names would be removed. But consider that their removal was not instantaneously done with their first sin. We don't know exactly when God removes a sinner's name, but there is some point where God clearly determines that the person is just not going to repent.
Regardless of when it happens, what is needed is clear. We need to be living a righteous life. When we sin, we cannot remain in it. We must repent and return to God (II Corinthians 7:10-11). Our rejection of sin is critical. We don't make a practice of staying in sin (I John 3:5-10).