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As Christians, we are to be leaven in the culture.
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Leaven is a picture of sin.
Unleavened is the goal of the Church.
But, it’s probably not an accident that God had Unleavened Bread for Passover and 2 leavened loaves at the Feast of Weeks as an offering, given the birth of the Church was at the Feast of Weeks.
Not always. There is bad "leaven" as well as good.
Jesus places the image of leaven as comparable to the mustard seed as something that invisibly causes the growth and spreading of the kingdom of God:
Jesus said therefore, “What is the kingdom of God like? And to what shall I compare it? It is like a grain of mustard seed which a man took and sowed in his garden; and it grew and became a tree, and the birds of the air made nests in its branches.”
And again he said, “To what shall I compare the kingdom of God? It is like leaven which a woman took and hid in three measures of meal, till it was all leavened.” (Luke 13:18-21)