5:3 For I most assuredly, as being absent in body but present in spirit,
have already, as though I were present, judged him who has done this thing.
5:4 In the name of our Lord Jesus Christ, you being gathered together,
and my spirit, with the power of our Lord Jesus Christ,
5:5 are to deliver such a one to Satan for the destruction of the flesh,
that the spirit may be saved in the day of the Lord Jesus.

5:6 Your boasting is not good. Don't you know that a little yeast leavens the whole lump?
5:7 Purge out the old yeast, that you may be a new lump, even as you are unleavened.

For indeed Christ, our Passover, has been sacrificed in our place.
5:8 Therefore let us keep the feast, not with old yeast,
neither with the yeast of malice and wickedness,
but with the unleavened bread of sincerity and truth.

5:11 But as it is, I wrote to you not to associate with anyone who is called a brother who
is a sexual sinner, or covetous, or an idolater, or a slanderer, or a drunkard, or an extortioner.
Don't even eat with such a person.

5:12 For what have I to do with also judging those who are outside?
Don't you judge those who are within?
5:13 But those who are outside, God judges.
"Put away the wicked man from among yourselves."(Deut 17:7)

1Cor 5:1,2  It is actually reported that there is sexual immorality among you, and of a kind that does not occur even among pagans: A man has his father’s wife. And you are proud! Shouldn’t you rather have been filled with grief and have put out of your fellowship the man who did this?

A sign of an immature Christian is one who divides over trivial things while tolerating things he shouldn't. The previous chapter was referring to judging the value of one Christian's service relative to another's. It is judging who is greatest among you. These cannot be determined until the Lord comes. But this chapter is dealing with judging blatant sin among Christians. And I do mean "blatant" (offensively conspicuous). We should be careful not to judge too quickly the motives of other Christians for it's easy to be wrong about things which can only be inferred, but if sin is blatant and measurable, it should be dealt with.

This is unpopular and seldom practiced in modern "feel-good" Christianity. Christians will allow the lawlessness of other Christians to continue on unchecked. More commonly if you rebuke another Christian for sin, they will consider that itself to be sin. They may even gather other Christians together to accuse you of unChristlike behavior, simply because you rebuked someone for sin.

The Corinthian church was proud of its toleration of sin. Like many Christians today, they were comfortable with sin in their midst. Here's a sign of immaturity. They were divided over things they should have been united on (like attitude towards spiritual leaders and spiritual gifts), and they united over things they should have divided over (sin in the church).

1Cor 5:3-5  Even though I am not physically present, I am with you in spirit. And I have already passed judgment on the one who did this, just as if I were present. When you are assembled in the name of our Lord Jesus and I am with you in spirit, and the power of our Lord Jesus is present, hand this man over to Satan, so that the sinful nature may be destroyed and his spirit saved on the day of the Lord.

"sinful nature" is literally the word "flesh" ("sarx"). That is, Paul intended for Satan to "knock his sarx off" - if you're familiar with the idiom. Paul himself had benefited from such an experience when the Lord gave him a vision of heaven. "To keep me from becoming conceited because of these surpassingly great revelations, there was given me a thorn in my flesh, a messenger of Satan, to torment me." 2Cor 12:7 And Paul had also delivered others to Satan for like purpose. 1Tim 1:20 "Of whom is Hymenaeus and Alexander; whom I have delivered unto Satan, that they may learn not to blaspheme" The intention is to have the person experience humiliation in some form so as to bring them to repentance or help them to maintain a proper level of humility.

"When we are judged by the Lord, we are being disciplined so that we will not be condemned with the world." 1Cor 11:32

Perhaps this is something you may warn your kids about the next time they rebel.

1Cor 5:6-8  Your boasting is not good. Don’t you know that a little yeast works through the whole batch of dough? Get rid of the old yeast that you may be a new batch without yeast— as you really are. For Christ, our Passover lamb, has been sacrificed. Therefore let us keep the Festival, not with the old yeast, the yeast of malice and wickedness, but with bread without yeast, the bread of sincerity and truth.

This verse explains a number of other verses dealing with the symbolism of unleavened bread.

Mt 13:33 He told them still another parable: "The kingdom of heaven is like yeast (leaven) that a woman took and mixed into a large amount of flour until it worked all through the dough."

"You were running a good race. Who cut in on you and kept you from obeying the truth? That kind of persuasion does not come from the one who calls you.  'A little yeast works through the whole batch of dough.'" Galatians 5:7-9

In contrast to these, the unleavened bread is that of sincerity and truth. Jesus represents this unleavened bread. For the communion service in which Jesus associated his flesh with the bread, was actually a celebration of the week of unleavened bread just before the passover.Yet how sincere are Christians today? How concerned about truth? Christians should not be involved in lying, slandering and falsifying information. They should not put on a false face or speak deceitfully.

It doesn't take much hypocrisy to corrupt the whole church.

1Cor 5:9-11  I have written you in my letter not to associate with sexually immoral people— not at all meaning the people of this world who are immoral, or the greedy and swindlers, or idolaters. In that case you would have to leave this world. But now I am writing you that you must not associate with anyone who calls himself a brother but is sexually immoral or greedy, an idolater or a slanderer, a drunkard or a swindler. With such a man do not even eat.

To associate with those who call themselves Christians but who live a sinful lifestyle is to condone their sin and thus be a participant in it. Furthermore in saying "anyone who calls himself", Paul is leaving open the issue of their actual salvation status. For what a person calls himself does not necessarily reflect what he actually is. But with regards to what kind of sins Paul is speaking of, it is apparent that these are blatant, obvious and of which they refuse to repent.  There are a number of places which list some categories of sins as Paul does here, such as in the next chapter (1Cor 6:9,10) and Eph 5:5,6 and Gal 5:19-21. But it is the sense of accepting these as one's lifestyle which is so offensive.

Sexual immorality is a rather obvious one, though in fact it is not uncommon for alleged Christians to engage in premarital sex and even cohabit. Don't associate with them. But likewise in the case for those greedy for money and those who swindle others out of money. And for those who characteristically slander others and for alcoholics, which implies also drug abusers. While none of these lists are comprehensive, the idea is don't associate with alleged Christians living a lifestyle of sin.

 Do not be misled: "Bad company corrupts good character." 1Cor 15:33

1Cor 5:12,13  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 man from among you." (Deut 17:7)

Paul, quoting Deuteronomy, shows an application of the Law.. The Law of Moses is not irrelevant to the Christian life. Though we are not bound to its regulations we are to infer appropriate applications from its commands.

Those outside the church already stand in judgement - a judgement that in fact they have to be saved from. For  "Whoever believes in him is not condemned, but whoever does not believe stands condemned already" John 3:18 and "Whoever believes in the Son has eternal life, but whoever rejects the Son will not see life, for God’s wrath remains on him." John 3:36 It is not our business to judge them. For they already stand condemned. Our business is to save them.

But as for those who allege to be saved, namely those alleging to be Christians, that's a different story. Christians are to judge other Christians. And forgiveness is not unconditional among Christians, but requires that the fellow Christian confess their sins, as Jesus said in Matt 18:15-17, or else be disassociated from.

The church has suffered much from the abuse of this concept. In earlier centuries churches often expelled Christians on an illegitimate basis over issues like baptism, some even being murdered. Though today we tend to face the opposite problem where institutional leadership is complacent with regards to this instruction forcing the saints to associate with alleged brothers living a lifestyle of sin. And predictably a little leaven leavens the whole batch.