Revelation 3:1-5
1 And unto the angel of the church in Sardis write; These things saith he that hath the seven Spirits of God, and the seven stars; I know thy works, that thou hast a name that thou livest, and art dead.
2 Be watchful, and strengthen the things which remain, that are ready to die: for I have not found thy works perfect before God.
3 Remember therefore how thou hast received and heard, and hold fast, and repent. If therefore thou shalt not watch, I will come on thee as a thief, and thou shalt not know what hour I will come upon thee.
4 Thou hast a few names even in Sardis which have not defiled their garments; and they shall walk with me in white: for they are worthy.
5 He that overcometh, the same shall be clothed in white raiment; and I will not blot out his name out of the book of life, but I will confess his name before my Father, and before his angels.
Charles Bing - Grace, Salvation, and Discipleship - I Will Not Blot Out His Name, Revelation 3:5
(Salvation) Interpretation: The true believer will not lose his salvation, or the believer who does not remain faithful can lose his salvation.
(Discipleship) Interpretation: The overcoming believer is assured of a secure future.
There are three promises in this verse, but the one most cited is the assurance about the Book of Life, which is taken as the register of all who are saved. Those who think all believers are overcomers say this is the Lord's promise that believers will never lose their salvation. Another interpretation found among those who think Christians can lose their salvation is that believers will not lose their salvation as long as they are faithful. This, of course, would also mean that they can lose their salvation if they cease to be victorious. Both of these (Salvation) interpretations equate the parallel promises about being clothed in white garments and having one's name confessed before the Father and His angels as simply promises of eternal salvation or events conterminous with eternal salvation.
The problems with taking these promises as only references to salvation have already been discussed, chief of which is that salvation would then depend on our performance instead of God's grace. The wearing of white garments is sometimes interpreted as believers clothed in the righteousness of Jesus Christ, an indication of their justification. But the immediate context (v. 4) speaks of those who have not defiled their garments and who are worthy to walk with the Lord in white. This clearly speaks of their works and faithfulness that makes them worthy of reward - to walk with Christ in white seems to denote a special companionship or privilege. Later in Revelation, white garments refer to righteous acts, not justification (Revelation 19:8, 14). We have shown that confessing one's name before the Father (and His angels) speaks of the reward of commendation, not justification (see the discussion on Matt. 10:32-33). What would angels have to do with one's justification anyway?
What, then, does the Lord mean when He says that He will not blot out the name of the overcomer from the Book of Life? It means exactly that. It is an assurance that this can never happen. The Book of Life should be understood in light of Daniel 12:1 as a record of God's elect people. Jesus is using a figure of speech known as litotes, an understatement that uses a negative to emphasize a positive. It is as if someone catches a limit of fish and is asked how he did. He might say "Not bad," by which he means "Fantastic!" Or if someone invites a friend to her Thanksgiving feast and says "You won't starve!" You get the point. (Some examples of litotes in Scripture include John 6:37, Acts 20:12: 1 Cor. 10:5; Heb. 6:10; Rev. 2:11). This promise is a strong overwhelming assurance enjoyed by those who overcome difficult circumstances, that in spite of any adversity on earth (even death, Revelation 2:10), they will never ever be excluded from the company of those who are saved eternally. We should not make it imply the opposite - that Christ would ever remove the name of a believer from the Book. The intent of the promise is to commend and to comfort, not threaten. There is even a hint of special honor because one's "name" stands for one's reputation, and the parallel promise in this passage is Jesus' confession of that name before the Father and the angels. Perhaps we could say that in addition to being listed forever, the faithful overcomer gets honorable mention in both written and spoken forms.
The Evangelism Study Bible - Revelation 3:5
3:5 "Book of Life": This verse is not a threat regarding loss of salvation but an affirming promise of salvation. It offers comfort that the names of believers will not be blotted out of the Book of Life (Revelation 13:8).