If we apply the Lord’s teaching that we must forgive one who sins against us repeatedly, whenever he comes to confess his fault (Mt 18:21–22), will we not encourage him to sin further and also to take advantage of our magnanimity? I am thinking, for example, of a compulsive gambler who repeatedly steals his wife’s money, looses everything and then asks her for forgiveness. What should we do in such a case?

When the Lord instructs us to forgive “Until seventy times seven [times]” (Mt 18:22), He is instructing us that we should never weary in forgiving. We must not set a limit on how many times we would forgive a person. As God forgives us freely of our sin against Him, so we must be ready to forgive others.

However, there are two things we must take note of in the Lord’s instruction, lest applying it wrongly we not only suffer hurt unnecessarily but bring dishonour to Christ by encouraging sin.

In the first place, note that the Lord’s instruction does not regardeveryone that trespasses against us, but the brethren in Christ that sin against us (Mt 18:15). If the Lord were speaking about everyone, why does He tell us to regard the unrepentant man as “an heathen man and a publican” (v. 17)? This fact immediately reduces considerably the possibility of abuse the way you have spelt out, for such a person as would abuse the Lord’s teaching would unlikely be found in a church that is faithful in discipline and doctrine.

In the second place, note that the Lord does not teach us to forgive unconditionally. We must always have a forgiving spirit or a readiness to forgive, by which we may pray for our enemies and not desire revenge even if they do not seek forgiveness (Mt 5:44). But that does not mean that we are to forgive and receive into favour and fellowship anyone who has unrepentantly sin against us, and therefore against the Lord. The Lord makes this clear in Luke 17:3–4:

Take heed to yourselves: If thy brother trespass against thee, rebuke him; and if he repent, forgive him. And if he trespass against thee seven times in a day, and seven times in a day turn again to thee, saying, I repent; thou shalt forgive him (emphasis mine).

Notice the qualification: “if he repent” (v. 3)? Now, it is true that the Lord goes on to suggest that if he says “I repent,” we are to forgive him (v. 4). But in view of what He just said (v. 3), we must take the words “I repent” as credibly reflecting genuine repentance rather than simply any mechanical intonations of the words. In the parable of the Unmerciful Servant, the servant has fallen down on his knees to beg his master for patience with him (Mt 18:26). Not withstanding the fact that his repentance was not genuine, his master “was moved with compassion” (v. 27) and forgave him. But once his repentance was discovered to be false by his ingratitude and refusal to forgive his fellow servant, he was thrown into prison (v. 34).

It appears, therefore, that when the Lord teaches us to forgive upon every verbal confession, He is teaching us to subdue our natural severity and suspicion of repeated offenders and be very ready to forgive, so long as we have the slightest reason to believe that the repentance is genuine.

Once again, Calvin’s pastoral remarks in this regard are most helpful:

… Christ does not deprive believers of the exercise of judgment, so as to yield a foolish readiness of belief to every slight expression, but only desires us to be so candid and merciful, as to stretch out the hand to offenders, provided there be evidence that they are sincerely dissatisfied with their sins. For repentance is a sacred thing, and therefore needs careful examination; but as soon as the offender gives probable evidence of conversion, Christ desires that he shall be admitted to reconciliation, lest, on being repulsed, he lose courage and fall back.

[Moreover], it must be observed that, when any man, through his light and unsteady behaviour, has exposed himself to suspicion, we may grant pardon when he asks it, and yet may do so in such a manner as to watch over his conduct for the future, that our forbearance and meekness, which proceed from the Spirit of Christ, may not become the subject of his ridicule. For we must observe the design of our Lord himself, that we ought, by our gentleness, to assist those who have fallen to rise again. And certainly we ought to imitate the goodness of our heavenly Father, who meets sinners at a distance to invite them to salvation. Besides, as repentance is a wonderful work of the Spirit, and is the creation of the new man, if we despise it, we offer an insult to God himself (Comm. in loc.).