13:33 leaven. Leaven is invariably symbolic of evil doctrine or practice. It produces fermentation, which is a decay process; decay and death, of course, were the precise features of God’s great curse on the earth because of sin (Genesis 3:17-19). The Levitical offerings, for this reason, could not be “made with leaven” (Leviticus 2:1,11), and the Passover memorial had to be observed with unleavened bread (Exodus 12:15). The spreading of leaven represents the corrupting influence of even a small amount of false doctrine (Galatians 5:9) or impure practice (I Corinthians 5:6) into the kingdom.