Before getting to the subject, look at Titus 1:15-16 and notice especially v16. "They profess that they know God; but in works they deny Him, being abominable, and disobedient, and unto every good work reprobate." 2 Timothy 3:5,8 and 13 tell us that in the last days there would be people "Having a form of godliness, but denying the power thereof (men of corrupt minds, reprobate concerning the faith) But evil men and seducers shall wax worse and worse, deceiving and being deceived."
These people claim to be saved, but deny by their works that they have ever had a genuine, life-changing experience. We encounter this more and more these days. On the contrary, look at Hebrews 6:9-12 and notice there are some "things that accompany salvation." Salvation never stands alone and there are some things that Genuine Salvation will change about a true believer. Look back at Heb. 4:1-2, "Let us therefore fear, lest, a promise being left us of entering into His rest, any of you should seem to come short of it. For unto us was the gospel preached, as well as unto them: but the word preached did not profit them, not being mixed with faith in them that heard it."
Look at 1 Cor. 6:9-11. "Know ye not that the unrighteous shall not inherit the kingdom of God: Be not deceived: neither fornicators, nor idolaters, nor adulterers, nor effeminate, nor abusers of themselves with mankind, Nor thieves, nor covetous, nor drunkards, nor revilers, nor extortioners, shall inherit the kingdom of God. And such were some of you: but ye are washed, but ye are sanctified, but ye are justified in the name of the Lord Jesus, and by the Spirit of our God."
Turn to Ephesians 6:1-17 and notice v5, "For this ye know, that no whoremonger, nor unclean person, nor covetous man, who is an idolater, hath any inheritance in the kingdom of Christ and of God. Let no man deceive you with vain word." James 1:22, "But be ye doers of the Word, and not hearers only, deceiving your own selves."
2 Cor. 5:17, "Therefore if any man be in Christ, he is a new creature: old things are passed away; behold all things are become new." Have you become a new creature? Real Christians are New Creatures!
2 Cor. 13:5, "Examine yourselves, whether ye be in the faith; prove your own selves. Know ye not your own selves, how that Jesus Christ is in you, except ye be reprobates?" Are you sure you are a Christian?