Did Matthew suggest that if a member of the church sinned against someone, they were to just quietly forgive them in their heart and say nothing? Not hardly! He gives a detailed process for actively coping with the problem. First go to the person and talk to them about it, one on one. If that doesn't work, then (2) take one or two others with you, so that there can be witnesses. If the member still will not listen, (3) tell the whole church. If they still refuse, (4) exclude them from the church community. Shun that person.
1Corinthians 5:3-5, 9-13 and 2 Corinthians 13:1-3 show that Paul used excommunication with this process. The Ananaias/Sapphira incident in Acts 5:1-11 also shows this to be standard in many early churches.
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Jesus, of course, sought out "Gentiles and tax collectors." But the early church was trying to establish its identiy and keep that definition of itself clear. Jesus was seeking lost sheep. Paul was dealing with wolves mutilating his sheep "in house" and he took aggressive action to prevent that.
The second half of the passage is the powerful rationale for this aggressive action againt people who remain outside the defining identiy of Christians. If that were allowed to continue, the result would destroy the harmony and unity that makes the community of Christians so powerful. "If they agree, they can ask anything and God will grant it. If they gather in Christ's Spirit, He will be there with them. The whole point of the church was to create and maintain harmony in love -- to learn to love one another very well, indeed. So it was essential not to gloss over, but to deal with and work through problems in relationships because the harmony of right relationships was the basis for the new order.
To avoid any misunderstanding, in the very next verse Matthew records the 7 times 7 admoniton from Jesus on forgiveness. Forgiveness is not optional for the Christian. But forgiving someone does not mean that they can retain their church membership if they are not repentent. This is our eye into the early church realizing that not anyone or everyone will be able to be in their group, without creating chaos or even destroying its identity.
The modern church is allergic generally to confrontation. Everyone should just "be nice." Ignore problems. The courage required to follow Matthew's 4 Step program has the potential though to resolve the problem, one way or another. It's goal is to bring the person back into harmony with the group. He doesn't suggest forbearance, endurance, patience in accepting bad behavior. Quite the contrary.
I read recently that most people carry about 10 lbs. of waste material around that has gotten trapped in their colons, where it ferments and eventually causes serious illness. They wash on the outside, but are filthy on the inside, the author said.
This is just as true on the spiritual plane, where silent martyrdom can cause an accumulation of festering sores. But speaking the truth to each other in love clears the atmosphere for forgiveness and change. It wishes well to the offender by giving opportunity for better behavior, rather than passive observation of denigration of the self and the group.