Boyd’s Bible Dictionary:

(raise). Old fermented dough used to lighten new dough (Matt. 13:33). Passover bread unleavened (Ex. 12:15-17). Corrupt doctrines (Matt. 16:6); evil passions (1 Cor. 5:7-8).

Concise Bible Dictionary:

Leaven (Yeast)
This was early used in the fermentation of bread. As a symbol it is always used in scripture for the working of the human element, whether mind or flesh, in the things of God, and hence evil. It was strictly forbidden to be burnt in any offering made by fire (Lev. 2:11); but in the peace offering, besides the unleavened cakes and wafers, the offerer was to present leavened bread, which was to be eaten (Lev. 7:12-13; Lev. 23:17-18). Its presence here might seem to suggest an exception to the statement that leaven always signifies that form of evil; but it is not, for the peace offering typifies worship, and there, alas, the worshipper is not entirely free from indwelling sin. In the parable of “the leaven hid in the meal,” it also represents the same evil, which in an insidious way permeates the mass with which it is mixed. The solemn words are added, “till the whole was leavened” (Luke 13:20-21). It is only a too true similitude of the kingdom of God, for everywhere evil is spreading therein. In Matthew 16:6-12 leaven is applied to the teaching of the Pharisees and Sadducees. In the church, leaven when discovered must be purged out, for “a little leaven leaveneth the whole lump” (1 Cor. 5:6-8); but in the kingdom it is represented as working until all is leavened (Matt. 13:33). It is then that the King will purge out from His kingdom all that offend and commit iniquity, and cast them into a furnace of fire.
Leavened Bread

From Manners and Customs of the Bible:

Matthew 13:33. The kingdom of heaven is like unto leaven.
See also Luke 13:21.
The usual leaven among the Jews consisted of dough in a high state of fermentation, though the lees of wine were sometimes employed.

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