“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. 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. 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. Truly, I say to you, whatever you bind on earth shall be bound in heaven, and whatever you loose on earth shall be loosed in heaven. Again I say to you, if two of you agree on earth about anything they ask, it will be done for them by my Father in heaven. For where two or three are gathered in my name, there am I among them.” (Matthew 18:15-20 ESV)
The person who sinned against must go to the offender. This step is the opposite of how we often respond to sin. We expect the offender to come grovelling to us asking for forgiveness. However, Jesus places responsibility on the person offended to go to the other. Often, people do not realize they have sinned against us. Approached properly, with respect and love, they will often respond with immediate repentance and restoration. These are true believers seeking to live in harmony with their brothers and sisters in Christ. Church discipline fails when we go to sinners with the goal of removing them or of hurting them. Church discipline must never be an excuse for vengeance.
Jesus made it clear by His words that the goal is to restore relationships not break them. Only when the person fails to respond to multiple efforts by the offended, a group of believers, and finally the whole church should it be assumed that the person perhaps is not a believer. In that case, there is no basis for Christian fellowship. The person should be treated as one who is lost and in need of salvation.