MATTHEW 5:21-25

Q:  Is Jesus saying here that when you sin against another person, it cannot be forgiven unless you confess to both that person and to God?

These particular verses may have to do more with practical consequences of disagreements with a brother (including the hindering of your relationship with God, as do poor relations with your wife—taught elsewhere) and the general idea that “murder” has a broader definition than popularly believed. It doesn't deal with confessing an unknown sin against a person of which he is unaware, but one which is already causing bad relations between you and a fellow believer.

There are two other things to keep in mind when reading these verses: (1) The teachings combine principles applicable to temporal punishment here on earth (“You will be liable to the council) and eternal punishments (You will be liable to the hell of fire). (2) The punishments mentioned are not sure to happen, but you will be liable to receive them.