In both the Old Testament and the New Testament leaven is a “type of evil teachings, evil doctrines and evil practices. It is always to be put away and cast out as an unclean thing. The gospel is never called leaven. Nothing good is ever compared to leaven. Nothing good is ever said about leaven. In every place it is mentioned, leaven is defiling and is to be put away. (See Ex. 12:15; Lev. 2:11; I Cor. 5:6; Matt. 13:33).” [Wilson, Walter Lewis. Wilson’s Dictionary of Bible Types; page 286.]
In Scripture, leaven denotes any substance used to induce fermentation in either meals, dough or liquids. “For fermentation is the result of the divine curse upon the material universe because of sin.” [Wuest, Kenneth S. Mark in the Greek New Testament; page 162.] And Genesis 19:3 is the first use of the term ‘unleavened;’ “But he insisted so strongly that they did go with him and entered his house. He prepared a meal for them, baking bread without yeast, and they ate.” Here, Lot is serving unleavened bread to his angelic visitors.
And Exodus 12:8, 15-20, first utilizes the term in connection with a feast day – Passover and the Feast of Unleavened Bread. In these verses leaven is presented as being totally rejected and symbolic of evil; whereas unleavened bread is a pattern or type of Christ and symbolic of his sinless perfection.
In Matthew 13:33, leaven portrays religious apostasy during the Tribulation. “He told them still another parable: ’The kingdom of heaven is like yeast that a woman took and mixed into a large amount of flour until it worked all through the dough.” " The woman is the apostate church, the meal is the Word of God, the leaven is evil and apostate teachings concerning the Word of God. In other words, the woman mixes false doctrines with true doctrines and thus poisons those who eat it." [Wilson, Walter Lewis. Ibid.]
In Matthew 16:6, leaven represents the sophistry (false arguments) of the Sadduccees, which sophistry resulted in apostasy. “‘Be careful,’ Jesus said to them. ‘Be on your guard against the yeast of the Pharisees and Sadducees.’” And the leaven of the Pharisees was the evil of legalism and ritualism (Mark 8:15; Luke 12:1). Note that the Sadducees refused to accept any doctrine that could not be validated by reason, i.e., they claimed the right of private interpretation of the Torah. This rationalistic approach resulted in the following heresies: the denial of the resurrection and recompense in hell for unbelievers, since the Sadducees claimed that the soul expired with the body; the denial of angelic beings; and the acceptance of fatalism.
Mark 8:15 alludes to the leaven of Herod, which is the sin of lust for power. “‘Be careful,’ Jesus warned them. ‘Watch out for the yeast of the Pharisees and that of Herod.’”
While the leaven of the Corinthians refers to the sin of antinomianism (Christian sect that held that faith ruled out the need for morality), sexual lewdness (fornication, homosexuality, incest, etc.), and phallic apostasy, I Corinthians 5:1,2,6,7. “Your boasting is not good. Don’t you know that a little yeast works through the whole batch of dough? Get rid of the old yeast that you may be a new batch without yeast – as you really are. For Christ, our Passover lamb, has been sacrificed.” (I Cor. 5:6,7).
In contrast to the Corinthians, the leaven of the Galatians relates to the sin of legalism (gaining salvation through good works), and in this particular instance designates salvation by means of circumcision, Galatians 5:9, “A little yeast works through the whole batch of dough.”
[Thieme, Robert. Ibid. First catalogued by Robert Thieme; altered and appended by R. E. Radic.]