When to use Matthew 18:15-17 Biblically

Many misapply scripture and Matthew 18:15-17 is no exception. This is speaking to a private matter for the local church body/fellowship to deal with, period. If the matter is already public, it does not apply.

Matthew 18:15 Moreover if thy brother shall trespass against thee, go and tell him his fault between thee and him alone: if he shall hear thee, thou hast gained thy brother. 16 But if he will not hear thee, then take with thee one or two more, that in the mouth of two or three witnesses every word may be established. 17 And if he shall neglect to hear them, tell it unto the church: but if he neglect to hear the church, let him be unto thee as an heathen man and a publican.” (Matthew 18:15-17)

Besides directly applying to a church context, it’s clear that these verses have to do with restoring a brother who has sinned without informing more people of his sin than necessary.  Paul didn’t do a Matthew 18 with the fornicator in the Corinthian church (1 Cor. ch. 5) because his sin was already public knowledge (“it’ is commonly reported”).  There is no need to hide from the church something they all know already anyways.  This is why when one sins against another without others involved they are to deal with that person individually before they take the matter to others.  Notice in verse 16 that when the matter is brought to 2 or 3 others, it is so that every word may be established before multiple people.  When there is enough known publicly already to give grounds for the charge, this is of course totally unnecessary.  And the last stage of bringing the matter before the church would not only be unnecessary, but impossible as well when there’s not a church to bring the matter to.  And when the public evidence is there and the church or whatever group they are a part of isn’t doing anything about it anyways, then the whole church or group is in sin like the Corinthians were in the 1 Corinthians 5 matter.  The only need to do a Matthew 18 is to establish things that aren’t a matter of public knowledge before involving more people in the matter.  When the rebuke is based solely on things done, said, taught, etc publicly then the function of the process in Matthew 18:15-17 has already been accomplished.

Adam Clark Commentary:
Tell it unto the Church – “Lay the whole matter before the congregation of Christian believers, in that place of which he is a member, or before the minister and elders, as the representatives of the Church or assembly.”
Reproving a brother who had sinned was a positive command under the law. See Lev 19:17. And the Jews have a saying, that one of the causes of the ruin of their nation was, “No man reproved another.”