Question: Leaven was mentioned in last Sunday’s lesson. Could you please discuss the use of leaven in the Bible? Does it always stand for something bad?
As far as I can tell, leaven carries a bad connotation in all its uses except for one in each testament of the Bible. So generally, (and all generalizations have exceptions) leaven pictures a silent, spreading corruption.
Ye shall bring out of your habitations two wave loaves of two tenth deals: they shall be of fine flour; they shall be baken with leaven; they are the first fruits unto the LORD.
(Lev 23:17)
In this NT parable, Christ uses leaven to describe the kingdom of heaven. Some insist that this is also a negative use of leaven, but I don’t follow their arguments. There is also the leaven that should be offered during the feast of first fruits.
Another parable spake he unto them; The kingdom of heaven is like unto leaven, which a woman took, and hid in three measures of meal, till the whole was leavened.
(Mat 13:33)
You Think Too Much! 
I found the argument that leaven is not good in Matthew 13 to be pretty convincing.
In every other analogy given there it shows legitimate growth mixed with sinful pretensions -the saint and hyprocrit together. The fowls in the mustardtree, the tares with the wheat, the seed on bad ground. The leaven fits with all the other parables as a type of sin/something bad.
And it works out in practice too – how much of the morass described as “Christianity” is the real thing? It fits the context and what we see around us.
Not trying to correct you here, just wondering what you disagree with in that.