15 “If your brother sins against you, go and tell him his fault, between you and him alone. If he listens to you, you have gained your brother. 16 But if he does not listen, take one or two others along with you, that every charge may be established by the evidence of two or three witnesses. 17 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 collector.” – English Standard Version
For many years I misunderstood this passage. Still today in many respected commentaries and Bible study guides1 this passage is framed quite differently than what Jesus taught which was according to accepted historical Jewish law.
Jesus speaks of this principle in the context of a church setting (GK: ecclesia). I was originally taught that based on Matthew 18:15-17 that if someone sinned against me that I should seek their restoration by going to them and lovingly share how they sinned against me, with the desire that they would own up to their actions and be spiritually restored. Restoration was the goal. If, however, they did not own up to it, then I had the right to bring into the conflict others who were spiritually mature and in good standing within the church to confront the offender. If they still did not repent, then the matter could be brought to the church as a whole (either before the congregation or the leadership of the church). If they still failed to accept responsibility they could be removed from the congregation.
On more than one occasion I became aware of such proceedings that rose to the level of congregational confrontation as well as several that resulted in being brought before the elder leadership of the church. Some ended well with eventual repentance and restoration. Others, not so well. Although in some instances efforts to bring restoration fit the instructions that Jesus laid out (having more than one witness); But more often than not, I have seen it fall short of the historical long-standing biblical principle Jesus was affirming. And I have often heard it wrongly taught.
This passage cannot be used if there are no witnesses besides the offended person

If an offense has occurred, it would be helpful to share with the offender the effect of their speech or behavior so as not to do it again, NOT however, because of Matthew 18:15-17, but out of a desire to help another brother or sister in Christ (Galatians 6:1).
Matthew 18:15-17 can only be applied when two conditions exist
1. Matthew 18 applies when unrepentant sin is involved. The passage begins with the conditional statement, “IF your brother sins against you….” This passage is referring to cases where clear identifiable wrongdoing against another in the church is involved. If sin is not involved, then this passage should not be used.
2. Matthew 18 applies when there are at least two or three witnesses of the sin, not the conversation between the two disputants. When witnesses to the sin do not exist, the process cannot move forward. (Dr. Ken Newberger, Ph.D; Th.M, Dallas Theological Seminary)
Misinterpretation
Some writers identify the two or three witnesses, not as eyewitnesses of the sinful act being charged, but as witnesses of the allegation. This concept of a witness of the allegation for confirming truth is nowhere found in the Bible, be it the Old Testament of which Jesus is quoting, or in the New Testament. See how the word “witness” is understood throughout the Bible. (1 Kings 21:5-14, Mark 14:55-64, Acts 7:8-14).
Dr. Newberger2 explains why this concept of a witness makes little sense. “For example, if Susan charges Debbie with sin and Debbie denies the allegation, what good are witnesses to the conversation? How would such a process protect against false accusations? Having a person be a witness of a conversation where a charge is leveled but not be a witness to the alleged wrongdoing does nothing to uncover or establish the truth. Such accusations cannot be acted upon.

Indeed, the protection against false witnesses was so important that it was incorporated into the Ten Commandments (Ex. 20:16, Deut. 5:20; see also Mk 10:19; Luke. 18:20). Even Jesus was afforded this protection at his trial (Mark 14:55-64).
Biblical Basis for the Correct Interpretation
In Matthew 18, Jesus was quoting from Deuteronomy 19:15
A single witness shall not suffice against a person for any crime or for any wrong in connection with any offense that he has committed. Only on the evidence of two witnesses or of three witnesses shall a charge be established.
A single eyewitness, was not sufficient evidence or testimony to charge a person with a crime or offense.

Numbers 35:30 (see also Deut. 17:6)
If anyone kills a person, the murderer shall be put to death on the evidence of witnesses. But no person shall be put to death on the testimony of one witness.
The Baker Encyclopedia of the Bible under the entry of “Witness” tells us:
“In the judicial procedure outlined in the OT one witness was not adequate for personal testimony against anyone, but two or three witnesses were required (Deut. 17:6, 19:15). This principle was ingrained in Jewish law and is reiterated in the NT (cf. Mat. 18:16, 2 Cor. 13:1).
In his scholarly commentary on Matthew, and specifically Mat. 18, Craig Keener3 states,
“although disciples seek reconciliation, they must gather evidence in the proper order in case they later need proof of what transpired (Mat. 18:16). The requirement of two witnesses remained standard judicial procedure in early Christianity (2 Cor. 13:1-2, 1 Tim 5:19-20).”
In the same way the disciples were to preach the gospel having been actual eyewitnesses to Jesus’ ministry on earth (Luke 24:36-48, Acts 1:21-22), so must the witnesses discussed in Mat. 18:16 be actual eyewitnesses to the wrongdoing that is being alleged. Without such witnesses, the process as outlined in Matthew 18 cannot proceed. Dr. J. Carl Laney4, speaking on this, wrote:
“In legal process, the absence of witnesses or the existence of only one witness makes securing a conviction difficult. The biblical requirement of additional witnesses safeguards the judicial process against false accusation, slander, and wrongful incrimination. In the disciplinary process, those who have observed a sin in the life of a believer are able to strengthen the rebuke by confirming the charges. The witnesses may also serve to bring new objectivity to the situation by helping the truth to surface, and could be called upon to testify if the case comes before the congregation.”

Similarly, the apostle Paul wrote in 1 Timothy 5:19:
Do not admit a charge against an elder except on the evidence of two or three witnesses.
John Stott5, in his commentary, wrote:
“That is, it must be substantiated by several people. In the Old Testament two or three witnesses were required to sustain a charge and secure a conviction, especially in regard to a capital charge. The same principle applies in New Testament times (e.g. 2 Cor. 13:1; cf. Mat. 18:16) in particular when Christian leaders are being accused. … This practical regulation is necessary for the protection of pastoral leaders, who are vulnerable to slander.”
Because of the seriousness of making a charge of sin against an elder of the church, the word of just one person is not enough. Two, and better, three people testifying to the veracity of the charge sufficiently confirm its truthfulness. Without such eyewitnesses, the charge should not even be entertained.
Summary. When the word “witness” is used in a legal sense, as in the formal church proceeding described in Matthew 18 and 1 Timothy 5, witnesses step forward to offer first-hand knowledge of events that occurred in the past. Being witnesses of an allegation being made after the fact are not the kind of witnesses of which the Bible speaks.
1 Example of a misinterpreted commentary in the ESV Study Bible: (Matt.18:16) Evidence of two or three witnesses follows the guideline in Deut. 19:15 and refers to witnesses of the subsequent confrontation described in this verse, not necessarily eyewitnesses to the original offense (Matt. 18:15).
2Dr. Ken Newberger, professor, former pastor and current Conflict Resolution practitioner with a Ph.D. from Dallas Theological Seminary.
3 Matthew by Craig S. Keener. IVP New Testament Commentary Series Volume 1, 1997
4 Dr. J. Carl Laney, Professor of Biblical Literature, Western Theological Seminary
5 John Stott, Professor, author and former Anglican pastor in the Church of England.