That being said, this question is certainly a legitimate one in the minds of many people. And this is likely due to the behavior of some Christians; however, it is also likely because of the behavior of many who think they are Christians or profess to be Christians, but who are not. The reasons many believe they are true Christians when they are not are many and varied. The false teaching that is so prevalent these days is certainly one reason. When churches eschew teaching sound doctrine, the end result will be congregants who do not know the truth of God's Word. How can they keep in step with the Spirit, when the Truth is not in them?
Also, some believe their recitation of a prayer or responding to an "altar call" alone may have turned them into a Christian. Many believe their religious traditions, such as being baptized as an infant, secured a spot in Heaven for them, or that their plentiful good works alone have put them in good standing with God. And of course some believe church attendance alone guarantees salvation. The point is that many who profess to be Christians are not Christians at all. Yet they complacently remain convinced that all is well with their soul. Sadly, many will live their entire lives believing they were Christians only to one day hear these words from Jesus Christ: "I never knew you. Away from me, you evildoers!" ().
The clear teaching of the Bible is that when someone is saved his life will most definitely change as he is a "new creation, the old has gone and the new has come" (). A true born again Christian will strive to bring glory and honor to Christ by living a life that is pleasing to God (; ). True saving faith will indeed produce works or "fruit" in the life of the believer (, ). Thus, if there are no works of love in one's life, a careful self-examination is certainly called for. The apostle Paul instructed those in Corinth to do this very thing: "Examine yourselves to see whether you are in the faith; test yourselves. Do you not realize that Christ Jesus is in you - unless, of course, you fail the test?" (). Indeed, any profession of faith that does not result in a changed life and good works is a false profession and such a person is not a Christian.
Now, even though the lifestyle of true Christians does reflect the presence of Christ in their hearts, we know we are not perfect. Christians do sin and the apostle John makes it clear that we deceive ourselves if we think otherwise (). And when Christians do sin, rest assured there are multitudes just lying in wait to use their "slip-up" to further denigrate the true body of believers. That is why Paul admonished the church in Thessalonica to abstain from even the appearance of evil (), and to live in such a way as to "win the respect of outsiders" (.
What Christians will not do, however, is engage in repeated or habitual sin (). One who engages in deliberate and habitual sin is simply proving that he does not know Christ and therefore cannot be abiding in Him even though he may live his life under the vast umbrella of religion and is thought, therefore, by many to be a Christian.
As believers mature in their faith, they will exhibit more and more evidence of their true Christian nature, such as their love for God, repentance from sin, separation from the world, spiritual growth, and obedient living. As Paul told the Romans, the genuine child of God has been set free from sin and has become a slave to God, and the result is eternal life ().