A Christian is a person who has, by faith, received and fully trusted in Jesus Christ as the only Savior from sin (Ephesians 2:8–9). That person becomes a "new creation" in Christ (2 Corinthians 5:17) and the Holy Spirit comes into the heart to dwell there forever (1 Corinthians 6:19; Romans 8:9–11). This new creation does not live like the old person once did, as a slave to sin. He is now a slave to righteousness and sin no longer has a hold on him (Romans 6:16–18). Do Christians still sin? Yes. But they hate their sin and want to be rid of it. They don't live in a manner inconsistent with their faith in Christ.
Fake Christians – Why are there so many?
Some Christians appear to be fakes because they are not really Christians at all. They may profess to be followers of Christ when they are really still "dead in trespasses and sins" (Ephesians 2:1) and have never been redeemed by Christ. They are living a lie. They profess Jesus, but they really love sin, and no amount of professing to belong to Christ will change their behavior and the fact that they prefer sin to holiness. They may also be deceived into thinking they are Christians because of something they have done, such as reciting a prayer, responding to an "altar call" or becoming baptized, none of which truly changes the heart as only Christ can.
Some may appear to be fake Christians because they believe that religious traditions, such as church attendance or being baptized as an infant, are sufficient to save them. Others confuse their good works with true conversion of the soul, even though God has declared all our works to be as "polluted garments" before Him (Isaiah 64:6). Though they may try to "clean up their act" in order to convince themselves, and others, of their spiritual state, they simply do not have the power to do so because the Spirit does not live within them. Naturally, when others compare their professions with their lives, they of course conclude that they are fake Christians when, in reality, they are not Christians at all.
The Bible is clear that when someone is saved, the new creation he has now become will be evident by a change in lifestyle. A true, born-again Christian will strive to bring glory and honor to Christ by living a life that is pleasing to God (1 Peter 1:15–16). True saving faith will always be evidenced by the fruit of the Spirit that lives in the heart. The "fake Christian" doesn't have the ability to produce the Spirit's fruit and continues to exhibit the works of the flesh (Galatians 5:19–26). As Jesus reminded His disciples, we know who they are by their fruits (Matthew 7:20). Therefore, any profession of faith that does not result in a changed life and fruit of the Spirit is a false profession, and the professor is not a Christian.
Of course, there are Christians, especially those who are young or immature in the faith, who continually struggle to obey God and resist sin. True Christians are not perfect, and failures, even habitual failures, do not necessarily mean the person is a fake. Christians do sin, and when we do, unbelievers point to this as evidence that we are fake. However, a Christian doesn't live in a state of continual, unrepentant sin, what the Bible calls "the practice" of sin (1 John 3:6–9). The practice of sin is the mark of a non-Christian, not simply an immature Christian. As believers mature in their faith, they will prove that they are new creations by their increasing love for God, continual repentance from sin, separation from the world, spiritual growth, and obedience to God's Word. Fake Christians, on the other hand, will eventually prove that they do not belong to Christ at all and should pay attention to the admonition of the Apostle Paul, who said we should examine ourselves to see if we are truly in the faith (2 Corinthians 13:5).
What are some signs of authentic, saving faith?
How can I recognize a false conversion?
Are all Christians hypocrites? Why is the claim that Christians are hypocrites so popular? Is it true?
Do not judge - Is that biblical? What does the Bible mean when it says we are not to judge others?
What does it mean for Christians to be in the world but not of the world?
Truth about the Christian Life