New Defender's Study Bible Notes
5:9 A little leaven. Compare I Corinthians 5:6, where the same sentence is used by Paul.
5:9 leaven. Here, as always, when leaven is used in a figurative sense, it obviously symbolizes evil doctrine (note Matthew 16:12, for example). Leaven, physically speaking, seems to improve the taste but actually is producing disintegration of the molecular structure of the foodstuff in which it is placed—also producing fermentation in sweet fruit juices. In this case, Paul uses leaven to represent the corruption of salvation doctrine by legalism, especially the insistence on circumcision.