I think the "leaven" of the New Testament is the "meme" of today, a behavior-altering, self-propagating idea. Just as genes are propagated through humans, so are memes. Memes, like viruses, can remain dormant for long periods, start off as small as a mustard seed and then rapidly infect a society, transmitted from one human to another.
Just as yeast expands the dough, so do memes expand through society as persons share the idea with others. Some memes explode exponentially into a population, as one person tells two, the two tell four, then the four tell eight, and after twenty replications a million have been infected. Some memes are good, others bad; they can be corrupting, or correcting, informing, concealing, inspiring, or enlightening; they can be suppressed, purged, or planted, and they can be used to subdue. Thus, the Kingdom is God is compared in the Bible to a meme, a spirit-altering idea, small as a mustard seed, which eventually becomes spread over the entire earth.
Joseph F. Alward
"Skeptical Views of Christianity and the Bible"