Exodus 12:15

Exo. 12:15. Concerning leaven. It was a most fit type of the corruption of the heart by reason of its sourness, and because of its infecting spreading nature, so that a little leaven leavens the whole lump (in which respect also it is a fit type of false doctrine, as Matthew 16:6; Matthew 16:11; Matthew 16:12), and because of its swelling nature, for the nature of corruption is to swell self, it radically consists in inordinate self-love, and primarily is manifest in pride and self-exaltation. The swelling nature of leaven represents the nature of corruption with respect to its principle, viz. inordinate self-love; and the sourness of it represents its nature with respect to its tendency, which is enmity.

But especially is leaven a fit type of original sin, by reason of the manner of its propagation; for as original sin is propagated from father to son, and so from generation to generation, so it is with leaven, one lump leavens the next, and that the next, and so leaven is propagated from lump to lump, forever. The old lump leavens the new, and therefore is called the old leaven.