“sins.” It is very important to pay attention to the word “sins” here. Far too often in the Church someone becomes offended at the behavior of someone else and makes it his or her job to “set them straight.” This verse is not about enforcing our opinions as to what is right, but correcting someone who is actually sinning in the eyes of God. While having friendly relations in the Church is important, there is a huge difference in helping people to not be offensive to each other and helping people to stop sinning.
“against you.” Early and important manuscripts of this passage omit “against you.” It is possible that scribes added the words “against you” to make it agree with what Peter said in Matthew 18:21, about a brother sinning “against me.” It is also possible, however, that the scribes copying the earlier manuscripts omitted the words to make the text have a wider application: i.e., that a person did not have to wait until someone sinned “against him,” he could intervene if he saw someone sinning. However, it was the tendency of scribes to add material to the text for clarity or harmony, rather than delete material to the text, making the shorter reading more likely to be original. Although most modern versions leave “against you” in the text, some modern versions omit the phrase (cp. GWN; NASB; NET; NIV2011; NJB; and Rotherham).
Whether the words “against you” are in the text or not, it is clear from the scope of Scripture, including verses such as Galatians 6:1, Ephesians 5:11, and 1 Timothy 5:20, that Christians do not have to wait until someone actually specifically sins against them personally before going to the person and pointing out the problem.
“go and tell him his fault.” This passage of Scripture in Matthew 18:15-17 gives four stages of action that a believer should take if there is someone in the congregation who is sinning. First, take the person aside and discuss it between yourselves. Second, take two or three other witnesses with you and discuss it together. Third, take the issue to the congregation, and fourth, if the person will not even listen to what the whole church congregation has to say, excuse that person from your company. The disciplined believer follows this pattern. Far too many believers are scandalized by the behavior of someone else and then go and tell lots of other people about it before they ever (if they ever!) tell the sinner to his or her face. If we want the Church to be a loving and godly place for people, we must follow Christ’s directives about how to handle problems.
We should also be aware that in many cases there is a fifth action, a very first action, that is not mentioned in the list of four actions here, and that is to overlook the sin if that can be done without compromising godliness (Prov. 19:11). Many times the best action to take when someone sins, especially since many sins are accidental and/or not habitual, is simply to overlook them. We sin too, and it would cause many hard feelings and be divisive in the Church if every time someone sinned another person tried to reprove him for it (cp. Ecc. 7:21-22).