Are you not to judge those inside (the church)?

Today's scripture reading gave me some new insight into when and whom I must exercise my judgement. Paul in 1 Corinthians 5:12-13 admonished us:
What business is it of mine to judge those outside the church? Are you not to judge those inside? God will judge those outside. “Expel the wicked person from among you.”

Now when I use the word "judge" I mean simply to give an opinion on a moral issue, often based on a combination of my understanding of what scripture teaches, and the reasonable application of the laws and customs of the church and society at the time. For the most part I try to avoid making judgement on what people wear or what music they like or whether they smoke cigarettes. I must admit it is often more difficult to draw the line on many other issues - like stewardship (with money and the environment), some behaviour (like disrespect for parents or parenting children), and even choice of vocation (I think most would agree that being a professional assassin is likely wrong!).

Here in chapter 5 of 1 Corinthians Paul was dealing with a case of incest in the church in Corinth. There are not too many specifics except this:
It is actually reported that there is sexual immorality among you, and of a kind that even pagans do not tolerate: A man is sleeping with his father’s wife. And you are proud! (verses 1-2)
The problem: a man is sleeping with his father's wife (sexual immorality)
The church's response: and you are proud!
The world's standard at the time: even pagans do not tolerate (it likely broke the existing law)

Initially I was inclined to think that this unnamed church member had married his young step-mother following his father's death, as a means to support the widow and perhaps to reproduce on behalf of the family line. The church was proud because it was seen as a self-giving act. Well it appeared not to be so. Apparently this may have happened while his father was still alive (2 Corinthians 7:12 gave a hint that there was "an injured party"). This act was in direct violation of the ancient law:
Do not have sexual relations with your father’s wife; that would dishonor your father. (Leviticus 18:8)
And dishonoring our parent is breaking the 5th commandment of the Ten Commandments.

Paul was also drawing a distinction between judging Christians and non-Christians. It is possible that the step-mother involved was a non-christian and Paul did not think it was his business to judge her (I think Paul was using those outside the church as to mean those who had not made a profession of faith).

So it seems that it is not only right for us to judge our own brothers and sisters in matters clearly in violation of God's laws and even the law of society, it is our responsibility so that God would be honored. Paul used the metaphor of the "bad yeast" that can ruin the entire dough in the context of the Passover lamb (Jesus) who made a costly sacrifice to redeem us (verses 6-8). This "little yeast" was an emblem of a church that had many other moral failures which Paul addressed in the books of Corinthians. We must deal with sins within the church seriously so that we can truly become the light to the world.