Any substance that will cause fermentation and will raise dough; also when introduced into the Mass modified and tempers the whole. Symbol of moral influence, whether good or bad. In the Jewish tradition unleavened bread is a sign of sinlessness. Christ spoke of the leaven of the Pharisees as harmful (Matthew 16; Mark 8). St. Paul urged the Christians to purge out the old yeast of evil and to have only the unleavened bread of sincerity and truth (I Corinthians 5:8). In one of the parables of the Gospel, the kingdom of heaven is compared to yeast that eventually leavens all who come under its influence (Matthew 13:33).