Jesus says, "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." (Matthew 18:15 ESV)
With this instruction in mind, here is an excerpt from "Running The Race Together Part 3–Confronting a Brother or Sister Who Has Sinned: An Exposition of Matthew 18:15":
It is really important to understand this relational, familial context for confrontation. The confrontation of Matthew 18:15 is not an act of judgment against outsiders whom we don’t know and who operate according to a different set of standards. These outsiders – these unbelievers – do indeed need to be confronted: they need to be confronted with the truth of the gospel, they need to be persuaded to turn from their sins and trust in Jesus. We need to love outsiders by going to them and proclaiming to them the good news of salvation. But Matthew 18:15 isn’t about the evangelization of unbelievers. Instead, this verse instructs us to hold our Christian brothers and sisters accountable to the path of discipleship, and to do everything we can to encourage them to be a loyal disciple who remains faithful to the Lord.
O Christians, behold your brothers and sisters! You are members of the same spiritual family! You have the same heavenly Father who cares for His people! You have the same Lord Jesus Christ who loved the church and gave Himself for her – and He came to rescue us from our sins! You have the same Holy Spirit who empowers the brethren to live a holy life! You have the same gospel of God’s grace, which is the sole cause of the salvation of sinners! You have the same treasure called the Bible, which reveals God to us and shows us the good path that He has set before us! It is one of your God-given responsibilities and privileges to help each other to stay true to the Lord. It is a family duty to – as some church covenants put it – ‘watch over one another in brotherly love’. It is a solemn obligation for faithful brothers and sisters to rescue their foolish siblings who have ignored the ‘No Trespassing’ sign and now find themselves in dangerous territory.