Whoop,
Look what you're referencing in each of these statements: a verse from the Bible.
I would disagree with Dor somewhat - Christianity does not "have nothing to do with going to Church," but at the same time, God's Word is the Truth, for the Word cannot change. *People* of a church are sinners by nature, and while they (most of them) strive to lead a better life, they will always fall short.
What I'm getting at can best be said by looking at the 2nd thing you said; Matthew 18:15 tells of how a person's continual sin should be handled, first by a single person, then by a small group, then by the church, etc. But, how can one person (or many) claim a person has sinned? Humans are all sinners, and there are none who are righteous. The Bible is the standard of Truth by which the Church acts.
I wish I was more knowledgeable about the Bible, or even had a better way of explaining what I'm getting at, but hopefully that made sense.