Please explain the parable of the leaven?

There are several places in the Gospels that use the image of “leaven.” For instance, Jesus tells his disciples to beware the leaven of the Scribes and Pharisees (see Matthew 20:6). In the context of that warning of Jesus, the “leaven” of the Pharisees refers to their corrupt religious practices and teachings, which led people astray by causing them to focus on the letter of the law rather than what was truly important. The passage also comes immediately after Jesus bestows authority on St. Peter as the “rock” of his Church. Jesus’ authority (and the authority of the Apostles) is being contrasted with the false authority of the Pharisees.

In Matthew 13:33, “leaven” is used in a different way by Jesus to describe the Kingdom of Heaven. Jesus uses many different images to describe the Kingdom and its nature (a mustard seed, wheat among weeds, etc.). Each parable and image tells us a little bit more about what the Kingdom is like.

In the case of this particular parable, the Kingdom is like yeast which “leavens” the bread because just a small measure of it causes the whole batch of dough to rise. In other words, the Kingdom may seem small, but it is incredibly potent. Just a small touch of God’s grace can raise to life the whole world. The fact that yeast makes dough rise also alludes to Christ’s Resurrection. Also, notice how the woman uses three measures of flour–three is a number that alludes to something special or sacred. And again, it refers to the Resurrection: Christ rose from the dead after three days.