Jesus said unto them, See ye and beware of the leaven of the Pharisees and of the Sadducees. Afterward the disciples understood that He had not said that they should beware of the leaven of bread, but of the doctrine of the Pharisees and Sadducees (Matt. 16:6, 12);

where "leaven" manifestly means false doctrine. As "leaven" signified falsity, it was forbidden to sacrifice upon what was leavened the blood of the sacrifice (Exod. 23:18; 34:25); for by the "blood of the sacrifice" was signified holy truth, thus truth. pure from all falsity (n. 4735, 6978, 7317, 7326, 7846, 7850). It was also ordained that the meat-offering which was to be offered upon the altar was not to be baked leavened (Lev. 6:17); and that the cakes and wafers also should be unleavened (Lev. 7:11-13). [2] As further regards what is leavened and what is unleavened, be it known that the purification of truth from falsity with man cannot possibly exist without fermentation so called, that is, without the combat of falsity with truth and of truth with falsity; but after combat has existed, and truth has conquered, then falsity falls down like dregs, and the truth comes forth purified, like wine which grows clear after fermentation, the dregs falling to the bottom. This fermentation or combat exists chiefly when the state with man is being changed, that is, when he begins to act from the good that is of charity, and not as before from the truth that is of faith; for the state has not as yet been purified when the man acts from the truth of faith, but it has been purified when he acts from the good which is of charity; for then he acts from the will; previously only from the understanding. [3] Spiritual combats or temptations are fermentations in the spiritual sense; for then falsities are desirous to conjoin themselves with truths; but truths reject them, and finally cast them down as it were to the bottom, consequently they purify. In this sense is to be understood what the Lord teaches about leaven in Matthew:

The kingdom of the heavens is like unto leaven, which a woman took, and hid in three measures of meal, until the whole was leavened (Matt. 13:33);

"meal" denotes the truth from which is good. Also in Hosea:

They are all adulterers, as an oven heated by the baker; the stirrer ceaseth from the kneading of the dough until it be leavened (Hos. 7:4). Because, as before said, such combats as are signified by fermentations arise with man in the state previous to newness of life; therefore it was also ordained that when the new meat-offering on the feast of the first-fruits was offered, the bread of waving was to be baked leavened, and was to be the first-fruits to Jehovah (Lev. 23:16, 17).