Believers can and do sin for extended periods of time. How can such people know whether their sin is a temporary failure or proof that they are unsaved?

“Obviously even in Scripture we see that believers sometimes sinned grievously and over long periods of time. David is one example (2 Sam. 11–12; Ps. 51); Lot is another (2 Peter 2:7–9). Christians who sin in such a fashion should not expect to enjoy assurance, however. Of course, true believers do not lose their salvation when they sin (cf. Rom. 8:35–39), but even David testified that he had forfeited the joy of salvation (Ps. 51:12).

When believers sin, they dishonor Christ (1 Cor. 6:15–17), they grieve the Holy Spirit (Eph. 4:30), and they subject themselves to the discipline of a loving Father (Heb. 12:5–7). If they can continue in sin without experiencing divine discipline, something is terribly wrong, as verse 8 explains: “If you are without discipline, of which all have become partakers, then you are illegitimate children and not sons.” 

Moreover, our Lord established a process for dealing with sin among believers:

If your brother sins, go and reprove him in private; if he listens to you, you have won your brother. But if he does not listen to you, take one or two more with you, so that by the mouth of two or three witnesses every fact may be confirmed. And if he refuses to listen to them, tell it to the church; and if he refuses to listen even to the church, let him be to you as a Gentile and a tax-gatherer. Truly I say to you, whatever you shall bind on earth shall be bound in heaven; and whatever you loose on earth shall be loosed in heaven. Again I say to you, that if two of you agree on earth about anything that they may ask, it shall be done for them by My Father who is in heaven. For where two or three have gathered together in My name, there I am in their midst (Matt. 18:15–20).

Notice that the discipline process Jesus outlined is specifically intended to answer the question of whether a person in sin is a true brother or an outsider. “If he listens to you [if he repents], you have won your brother” (v. 15). But ultimately, “if he refuses to listen even to the church, let him be to you as a Gentile and a tax-gatherer” (v. 17)—that is, regard him as an unbeliever and pursue him evangelistically. The Lord goes on to state that he personally mediates his rule on earth through this process (v. 20).

No one who persists in willful, deliberate sin and rebellion against the Lord should be encouraged with any promise of assurance. If you know someone like that who professes faith in Christ, follow the process of Matthew 18 and call that person to repentance. But do not encourage him or her with the promise of security. Such a person may be clinging to a false hope.”

-Pastor John MacArthur,
answering Common Questions about Lordship (Part 1)