It says that if you truly know that someone is wrong in there actions to bring it to there attention in a humble personal manner. If they refuse to change let them act out in front of others and confront them about it together. If that does not work bring it up in the church which is supposed to be unbiased and know right from wrong. If this still doesn't work you have the right to run them off or kill them. Not sure which would have to clarify the offence of tax collector. 18 is simply saying that if you are with the truly right your judgments will be upheld in the afterlife. 19 implies that the more people that believe the greater the effect will be.
Hope that helped.
Hope that helped.
