Who Is An Unbeliever?
Unbelief can cause one to be eternally lost. The Bible says, "And to whom did God swear that they would never enter His rest if not to those who disobeyed? So we see that they were not able to enter, because of their unbelief" (Hebrews 3:18-19). One who does not believe God cannot enter into God’s rest of Heaven, but will be cast into the eternal torments of Hell. When one does not obey the Lord’s commands, God counts this as unbelief. Jesus says, "But why do you call Me Lord, Lord, and not do the things which I say?" (Luke 6:46). If one doesn’t keep God’s commandments, this shows that one doesn’t really believe God. We read in 1 John 2:4, "He who says, ‘I know Him’, and does not keep His commandments is a liar, and the truth is not in him." One who says he knows God, but does not keep His commandments is not only lying to others but also to himself.
We are warned, "Beware, brethern, lest there be in any of you an evil heart of unbelief in departing from the living God" (Hebrews 3:12). A Christian becomes an unbeliever when he departs from God by not faithfully serving Him. God says, "But the cowardly, unbelieving, abominable, murderers, sexually immoral, sorcerers, idolaters, and all liars shall have their part in the lake which burns with fire and brimstone, which is the second death" (Revelation 21:8). One doesn’t have to do anything to be lost, except to be an unbeliever. One may claim to be a believer, but if he doesn’t faithfully obey God, he is an unbeliever. If one doesn’t believe God, he will not obey Him. Jesus is only "the author of eternal salvation to all who obey Him" (Hebrews 5:9).