In 1 Corinthians 5, Paul is urging the Corinthians to take action regarding a sexually immoral brother within their midst. Paul tells us that this person is engaging in a relationship that is not even condoned among the Greco-Roman culture of the day: a man is with his step-mother. It seems from what Paul writes that the Corinthians are not only condoning this behavior but actually boasting about it. While we may find this to be an odd thing for a group of Christians to boast about, it fits with the picture of the Corinthians we have gathered from the first several chapters of this letter. This is a congregation that thinks of themselves as so spiritual that they have been liberated from any sexual norms. They consider themselves to be "beyond" something as merely physical as sexuality. They likely regard this perverse relationship as proof of just how advanced they are in the ways of the spirit.
Paul says to the contrary "Your boasting is not good. Don't you know that a little leaven leavens the whole lump?" This saying was probably a popular proverb in Jewish culture similar to our saying "A bad apple ruins the whole bunch." Instead of seeing this behavior as something that the Corinthians can be proud of or even something that can simply be ignored, Paul says that it is a spiritual corruption which will spread throughout the entire community if it is left unchecked. It is worth noting here that Paul's address is to the church at large rather than to the immoral individual. This reinforces the notion that this immoral relationship is not merely an individual matter. It involves the health of the entire community.
However, Paul is not only making a point about the communal nature of their salvation. He is also making a point about the age or season, the epoch of history in which that salvation is taking place. Paul goes on to say "Clean out the old leaven in order that you might be a new lump of dough, just as you are unleavened. For Christ, our passover lamb, has been sacrificed." What does leaven have to do with Christ's death? The imagery here is that of the passover feast. Every year faithful Jew celebrated the passover meal as a reminder of God's deliverance of the people of Israel from their slavery in Egypt. One of the main parts of that meal was the eating of unleavened bread. It was unleavened as a symbol of Israel's hurried exit from Egypt... they had to be ready to go at a moment's notice, unable to wait around for bread to rise. Since unleavened bread was a part of the passover meal every year, it was an important custom to entirely cleanse one's house of all leaven before beginning to prepare the passover meal.... a kind of original spring cleaning. If you didn't, then there might be leaven on the surface where bread was prepared and the bread that was to remain unleavened might be contaminated with leaven.
By saying that Christ, our passover lamb, has already been sacrificed Paul is, in effect, telling the Corinthians that a new season in their lives has already begun but they have failed to carry out the appropriate cleansing in their lives for which that change in season calls. Christ's death and resurrection has inaugurated a new age in history, a new exodus, in which those who trust in Christ are set free from their slavery to sin. The Corinthians, however, are still living with the leaven of the old age in their lives by way of their boasting and sexual immorality. They must cleanse themselves of these old way of doing things in order to truly join in the celebration of their deliverance from the old way of life.
All of this leads Paul to make a clarification about something he said in an earlier letter to the Corinthians (now lost to us) which relates to the matter at hand. In that letter, Paul had told the Corinthians not to associate with sexually immoral people. It seems some in Corinth took this to mean that they couldn't have any contact whatsoever with anyone who didn't believe and behave as they did. Paul says that such a notion would be truly absurd; it would mean that you have to remove yourself from this world entirely. Paul now clarifies that the Corinthians should not associate with anyone who calls himself a brother and is sexually immoral - precisely the situation he is addressing in this chapter.
This chapter concludes with a statement which is an exceedingly apt principle for the Church to ponder today. Paul says "For what have I to do with judging outsiders? Is it not those inside the Church you are to judge? God judges those outside." In short, Paul's principle is this: hold those within the community of faith accountable for their actions but leave all other judgment up to God. Unfortunately, too much of the Church often does the exact opposite of this. We judge the culture around us for not living up to our values while we ignore and excuse those within our own fellowship who openly reject the life Christ calls us to while still calling themselves "Christian". If we are to be a people of this new season, this new exodus where we have been set free from the way of the old age by Christ's death and resurrection then we must not turn a blind eye to the sin and spiritual laxity within our midst.