Hastings' Dictionary of the Bible (1898)
LEAV'EN, a ferment mixed with dough to make it light, or a piece of dough or bread thus mixed and used to lighten a larger mass. Ex 12:15. It makes a thorough change in the whole, and hence the force of the parable. Matt 13:33, by which the silent influence of the gospel on the heart of man is beautifully illustrated. And so also it figuratively denotes the influence of false and corrupt doctrines, Matt 16:6, as well as the evil passions of the
depraved and unregenerate heart. 1 Cor 5:7-8. The Jews were forbidden to offer leaven and honey in the temple. Lev 2:11, and during the seven days of the Passover leaven was not even allowed to be found in their houses; whence the festival was often called the "feast of unleavened bread." Comp. 1 Cor 5:6.