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).