A “believer” is a child of God, born again by faith in Jesus Christ (see Acts 2:44); and an “unbeliever” is someone who does not believe in Jesus. In the New Testament, the Greek word apistou is translated “unbeliever.” Its adjective form literally means “not faithful,” and it describes someone lacking in Christian faith.
There is a clear distinction throughout Scripture between those who follow Christ and those who do not follow Him. The word unbeliever was understood to represent everything opposed to Christian faith and godliness. In 2 Corinthians 6:14–15, Paul writes, “Do not be bound together with unbelievers; for what partnership have righteousness and lawlessness, or what fellowship has light with darkness? Or what harmony has Christ with Belial, or what has a believer in common with an unbeliever?” The understanding was that those who had come to faith in Christ were in the process of being transformed into the image of Jesus (Romans 8:29). The further along they were in this process, the fewer similarities they had with unbelievers.
In regards to salvation, the word believe always implies far more than mental agreement with truth (John 3:16; 14:1; 1 John 3:23; Acts 8:37). Satan and the demons know more than any of us the facts about Jesus, the meaning of His death and resurrection, and the truth of Scripture (James 2:19). To believe for salvation (Romans 10:10) requires more than that. A believer is one who has transferred ownership of his life and future to the lordship of Jesus. Jesus said, “You are my friends if you do what I command you” (John 15:14). So a believer, in the biblical sense, is one who has responded to God’s calling and chosen Jesus as the Lord of his or her life. A believer looks to Jesus alone for salvation from sin and hell (John 14:6; Matthew 10:28; Romans 10:13). By the same token, an unbeliever is someone who has not allowed Jesus to be the Lord of his or her life. Religious-sounding words may have been spoken, but if the heart has not changed, it is not saving faith (Matthew 7:21–23). The person claiming salvation while lacking evidence of the transforming power of the Holy Spirit is still functionally an unbeliever.
When Scripture compares believers with unbelievers, it highlights the lifestyle differences. Believers are to be known by their love (John 13:35), their holiness (1 Peter 1:15), and their desire to be like Christ (Ephesians 4:15). According to the whole of Scripture, it is not possible to be a true believer and continue the ungodly lifestyles that warranted God’s wrath in the first place (Ephesians 5:5–7). This is due to the fact that salvation results in a new nature (2 Corinthians 5:17). Just as a fish cannot remain on land for long due to its nature, so born-again hearts cannot remain in sin for long due to their new nature.
First John 3:6 spells it out quite clearly: “No one who lives in him keeps on sinning. No one who continues to sin has either seen him or known him.” Then verses 8–10 clarify it even further: “The one who does what is sinful is of the devil, because the devil has been sinning from the beginning. The reason the Son of God appeared was to destroy the devil’s work. No one who is born of God will continue to sin, because God’s seed remains in them; they cannot go on sinning, because they have been born of God. This is how we know who the children of God are and who the children of the devil are: Anyone who does not do what is right is not God’s child, nor is anyone who does not love their brother and sister.”
“Without faith it is impossible to please God” (Hebrews 11:6); therefore, unbelievers cannot please God. They are still dead in their transgressions and sins (Ephesians 2:1). Unbelievers still bear the full weight of all their sin, and the “wrath of God remains on them” (John 3:36). They must be reconciled to God through Christ (Romans 5:10). Faith makes all the difference. Only through faith and surrender to the lordship of Jesus Christ can unbelievers become believers.