Christians should not sin presumptuously. The Bible tells us plainly to “shun the very appearance of evil". Of course, true believers do not lose their salvation when they sin (read Romans 8:35-39), but even David testified that he had forfeited the joy of salvation (Psalm 51:12). Continuous, deliberate sin will also lead to reprobation. This is a very dangerous position for a believer.

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

As for how to deal with a sinning believer, our Lord established a step-by-step process: (I like to refer to it affectionately as “the grievance procedure”)

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, My Father who is in heaven shall do it for them. For where two or three have gathered together in My name, there I am in their midst. (Matthew 18:15-20)

Notice that the discipline process Jesus outlined is specifically intended to determine whether a person in sin is a true brother or an outsider. “If he listens to you (i.e. 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 that process (v. 20).

No true believer will persists in willful, deliberate sin and rebellion against the Lord. 5And you know that He was manifested to take away our sins, and in Him there is no sin. 6Whoever abides in Him does not sin. Whoever sins has neither seen Him nor known Him. (1 John 3:5-6)

I love Christ, but I struggle constantly with sin in my life. Should I doubt my salvation?

No. Even the apostle Paul experienced perpetual struggle with sin throughout his life (Romans 7:7-25). All of us struggle continually with sinful thoughts, sinful attitudes, sinful habits, and sinful desires. It is those who don’t struggle—those who deliberately and eagerly revel in their sin—who need to have their sense of security shaken. So don’t give up in defeat if you find yourself struggling with sin or even if you fall into it. Stay in the battle.

Fight sin by considering deeply what makes you more vulnerable to it, and then establish godly practices in its place. You may find you need to have accountability with a godly person you trust, or spend more time in prayer, or stay away from things that tempt you. Do whatever it takes to discipline yourself and stop any pattern of sin before it gets started again. That is the key to staying away from sin—the constant, daily practice of thinking on the right things and not making provision for the flesh and its desires (Romans 13:14).

Can believers sin for extended periods of time? And 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 sin grievously and over long periods of time. David is one example (2 Samuel 11-12; Psalm 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. Romans 8:35-39), but even David testified that he had forfeited the joy of salvation (Psalm 51:12).

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

As for how to deal with a sinning believer, our Lord established a step-by-step process:

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. (Matthew 18:15-20)

Notice that the discipline process Jesus outlined is specifically intended to determine 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 that 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 don’t encourage him or her with the promise of security. Such a person may be clinging to a false hope.