Ephesians 4:14 indicates that even teachings from the Bible may become a kind of leaven if used improperly. "That we may be no longer babes tossed by waves and carried about by every wind of teaching in the sleight of men, in craftiness with a view to a system of error." We must realize that a wind of doctrine can corrupt the church life. It is not wise to emphasize doctrine too much or we will become leavened.
Philippians 1:15a says, "Some indeed preach Christ even because of envy and strife." Even with the preaching of Christ there could be some leaven. The preaching of Christ is good, but the envy and strife is a kind of leaven. Then verse 17a says, "But those announce Christ out of rivalry, not purely." Here is the preaching of Christ plus rivalry as a kind of leaven.
The book of Colossians also develops the matter of the leaven. In 2:8 it says, "Beware that no one carries you off as spoil through his philosophy and empty deceit, according to the tradition of men, according to the elements of the world, and not according to Christ." The philosophy of men is a kind of leaven. Colossians 2:18-19a says, "Let no one purposely defraud you of your prize, in humility and worship of the angels, standing on things which he has seen, vainly puffed up by the mind of his flesh, and not holding the Head." Even not holding the Head is a kind a leaven. We all must hold the Head, Christ.
Verses 20b-23 continue: "Why as living in the world do you subject yourself to ordinances: do not handle, do not taste, do not touch, things which are all for corruption in the using, according to the commandments and teachings of men? Which things indeed have an appearance of wisdom in self-imposed worship and humility and severe treatment of the body, but are not of any value against the indulgence of the flesh." The self-imposed humility, the worship of the angels, and the appearance of wisdom are a kind of leaven.
We can also see the leaven in 2 Thessalonians. In 2:2 we read: "That ye be not soon shaken in mind, or be troubled, neither by spirit, nor by word, nor by letter as from us, as that the day of Christ is at hand." In the ancient times, some pretended to be authorities in speaking or writing something concerning the Lord's coming back. This is a kind of leaven. "Let no man deceive you by any means: for that day shall not come, except there come a falling away first, and that man of sin be revealed, the son of perdition" (v. 3). All of the means of deceiving the Lord's children are a kind of leaven.