The Parable of the Leaven was given by Jesus in the New Testament (Matthew and Luke). It consists of essentially a single sentence, and immediately follows The Parable of the Mustard Seed in both Matthew and Luke: In Jesus' day, leaven was yeast or sourdough in a state of fermentation. When mixed into the dough, leaven causes the bread to rise when baked. The traditional teaching of the Church, both Catholic and Protestant, is that the Parable of the Leaven, like the preceding Parable of the Mustard Seed, shows how something small becomes something big.