Leaven

In the biblical symbol-system, leaven is amoral — neither good nor evil in itself. It represents doctrine, teaching, knowledge, influence: the system that permeates whatever it enters and transforms it from within. The type of leaven matters; “the leaven of the Pharisees” is their doctrine, not leaven as a category. Synonyms: yeast, leavening.

See A Lesson From The Days of Unleavened Bread