The 'Leaven' has an important biblical connotation. It is also referred to as the 'yeast' . The Israelites had been using it to make soft bread in Egypt. The leavened bread, was soft and round. The 'unleavened bread' was hard and flat. When they were delivered from the Egyptian bondage, by the strong arm of the LORD, they were commanded to eat unleavened bread. They had to leave Egypt in haste, and the LORD instructed them not to take the leaven from the land of slavery. They were asked to celebrate a week-long festival of the 'unleavened bread' in the wilderness. It was to remind them every year, of the hasty manner in which they had to leave Egypt and delivered by the LORD himself, out of the bondage. Later when they made their own leavened bread during their sojourn in the desert, they were allowed to offer it to the LORD and also to eat it.
Lord Jesus warned his disciples, to beware of the leaven of Herod, Pharisees and Sadducees. Why?
He meant that the leaven-like effect of the life of these Jewish sects, should not be allowed to corrupt the witness of his followers. The 'leaven' has a fermenting effect on anything in which it is mixed. Minute amounts of it is enough to ferment a big lump of dough. Its action is invisible, quiet but sure. On the exterior the leaven looks beneficial and user-friendly. Elsewhere in the gospels, the leaven is also referred to in a positive note, in that it spreads easily and into any larger amount. Jesus used it to teach the hearers, about the spreading influence of the Kingdom of God.
An attempt is made in this blog of mine, to expound what is meant by the caution served by our Lord Jesus Christ to his disciples, so that the body of Christ now, heeds the warning.