So far in Matthew 18 Jesus dealt with those who cause a little child to stumble (18:6-9) and the person who has wandered away from their faith (the one who has stumbled, 18:10-14). In this paragraph, Jesus addresses another related issue, a brother or sister in Christ who has some fault but has not yet stumbled and wandered away from the faith. As suggested by Craig Keener, this person may very well be a stumbling block to others, therefore they need to be addressed before they cause others to stumble (Matthew, 452).
Jesus is concerned his followers should discreetly confront those who are beginning to wander and bring them back into the flock as gently as possible. The earliest communities were very small house churches in the Jewish diaspora. As more gentiles were attracted to the Gospel, it is likely these house churches had to deal with serious conflicts between disciples of Jesus.
There are several difficult issues in 18:15-20. Jesus appears to lay down a process for church discipline, and Matthew’s use of church seems anachronistic. There was no church prior to the resurrection, although it is possible the word can refer to the community of Jesus followers, something like the yahad at Qumran. If this is the case, is there a direct application of the process to modern Christians as they confront one another over their faults?
The disciple of Jesus should attempt to deal with personal offenses privately. The verb is the typical one expected for sin (ἁμαρτάνω, aorist subjunctive). There is nothing here which implies this is offending someone’s preferences. For example, this is not about confronting someone for wearing a bolo tie and cowboy boots to church, nor is this about coming to church with a face full of piercings and tattoos. These things are matters of (good or bad) taste and not personal sin which is damaging to one’s spiritual life.
The phrase “point out their fault” (ESV) is a single word, ἐλέγχω. This verb can have the connotation of “to scrutinize or examine carefully, bring to light, expose” (BDAG) as in Ephesians 5:13. But is sometimes used for “express strong disapproval of someone’s action” (BDAG), to reprove or to correct someone (as in 2 Tim 4:2, “reprove, rebuke, and exhort, with complete patience and teaching”).
The problem is how this works out in real life. Some people really do enjoy pointing out another person’s fault. The Internet is full of people who have nothing better to do than argue about theological issues and condemn someone’s practice of their Christian faith. Some of those issues may be legitimate, but most of the time there is condemnation without any real engagement.
In the Sermon on the Mount Jesus talked about how one confronts a fellow disciple because of a sin. The disciple ought to deal with their own sin, the “log in your own eye,” before confronting someone over a minor sin, the “speck in their eye.” If the disciple of Jesus is busy dealing with their own spiritual life, then they will “see clearly” (Τότε διαβλέψεις). In Matthew 7:3-5 Jesus says the one who has dealt with a particular problem is able to gently correct a fellow disciple. To “gently correct” is the guiding principle in Matthew 18:15 as well.