1 Corinthians 5:6
6 Your glorying is not good. Know ye not that a little leaven leaveneth the whole lump?
This word glorying means boasting. Earlier, Paul accused them of being puffed up (4:6, 18, 19). He means they were inflated. Being puffed up is being prideful. Pride comes from boasting, which he says is not good.
Remember that it had been only 20 years or so since Paul’s conversion and his commission by the Lord that he would “bear my name before the Gentiles.”
Paul went away to Arabia for three years. When the apostles figured out what to do with the new Gentile believers, it was A.D 46. By the time Paul got to Corinth for the first time, established the church, and began teaching them daily for 18 months, it was A.D. 54. First Corinthians was written five years later in A.D. 59. The point being, there would have been more Jews in the church since Paul’s ministry to the Gentiles was still in its infancy.
The Jewish contingency of believers would have spread much more rapidly since they were the ones (Jews of the dispersion) who heard the word in Jerusalem during Pentecost in A.D. 33. At Pentecost, most were Jews and proselytes (Acts: 9,10).
Therefore, Paul uses a Jewish figure of speech to illustrate the problem. Know ye not is literally, you can see for yourself when baking bread, a little leaven leaveneth the whole lump. Just a pinch of leaven causes the whole loaf to rise, or get puffed up. This is the same effect pride will have in the church.
What should they do about it?