A Brother Who Sins Against You
"If your brother sins against you, go and show him his fault, just between the two of you. If he listens to you, you have won your brother over. But if he will not listen, take one or two others along, so that 'every matter may be established by the testimony 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, treat him as you would a pagan or a tax collector. Matthew 18:15-17
This is about resolving disputes between believers. You talk to the one who wronged you, but if that doesn't resolve it, you take it before a couple other believers. If that doesn't bring resolution, you bring it before the entire body of believers, who then can ostrasize him, if that's what it takes.
I can't understand what this has to do with fish on Friday rules and Jesus abdicating His authority..
Matthew 18:15f has to do with any contention between Christians, including whether or not to fast in particular manner on a particular day. The Church is given as a final arbiter.