One of the most striking ways the Bible teaches us concerning the power of influence is with the illustration of leaven. In each place it is used with reference to the pervading power of influence. Leaven is a substance that causes fermentation, like yeast. When only a small amount is added to a large lump of dough it ferments the whole lump. Though it is small in quantity it permeates the whole substance with which it comes in contact.
Influence can be either for the good or bad.
The kind of influence we have on others is based on what we allow to influence ourselves. If we are not careful some influences may be slowly corrupting and changing us. One can see the influencing power of leaven by the way that it changes the dough. When leaven is allowed to come in contact with the dough, even though the change is slow and gradual, it eventually changes that entire lump completely. The dough's shape, temperature, and smell are all affected.