Matthew 13:33 - The Parable of Leaven.

The kingdom of heaven is like yeast that a woman took and mixed
into a large amount of flour until it worked all through the dough.
Mat 13:33 (NIV)

Whereas the parable of the mustard seed traces the progress of
the Gospel in the world, the parable of the leaven shows the
progress of the kingdom in the heart and life of a believer.  It speaks
of the transformation of life from a small beginning in grace and
the effect of the Gospel on the total person.  Some have maintained
that the New Testament references to leaven always symbolize evil,
and therefore the parable describes the church being infiltrated and
pervaded by corruption and false doctrine. The immediate context
militates against the view that leaven is always a symbol of evil.  Jesus
indeed warned His disciples to "beware of the leaven of the Pharisees
and the Sadducees" (Mat.16:6).  But He also used leaven to describe
the growth of His kingdom as in the present instance.  The effect of
the leaven may be understood in two ways: As a process in general
history, leaven speaks of the silent yet pervasive and irresistible
working of the Gospel.  But just as leaven changes bread dough in an
observable way, so the parable also portrays the real and radical
transformation of individual lives through the Gospel.  [Daily Devotional
Bible]

The preaching of the gospel works like leaven in the hearts of
those who receive it. The leaven works certainly, so does the word,
yet gradually. It works silently, and without being seen, Mark
4:26-29, yet strongly; without noise, for so is the way of the Spirit,
but without fail. Thus it was in the world. The apostles, by
preaching the gospel, hid a handful of leaven in the great mass of
mankind. It was made powerful by the Spirit of the Lord of hosts, who
works, and none can hinder. Thus it is in the heart. When the gospel
comes into the soul, it works a thorough change; it spreads itself
into all the powers and faculties of the soul, and alters the
property even of the members of the body, Romans 6:13. From these
parables we are taught to expect a gradual progress; therefore let us
inquire, Are we growing in grace? and in holy principles and habits?
[Matthew Henry]

In the Saviour's parable, leaven is used to represent the
kingdom of heaven. It illustrates the quickening, assimilating power of
the grace of God.
None are so vile, none have fallen so low, as to be beyond the
working of this power. In all who will submit themselves to the Holy
Spirit a new principle of life is to be implanted; the lost image of
God is to be restored in humanity.
But man cannot transform himself by the exercise of his will. He
possesses no power by which this change can be effected. The
leaven--something wholly from without--must be put into the meal before the
desired change can be wrought in it. So the grace of God must be
received by the sinner before he can be fitted for the kingdom of glory.
All the culture and education which the world can give will fail of
making a degraded child of sin a child of heaven. The renewing energy
must come from God. The change can be made only by the Holy Spirit.
All who would be saved, high or low, rich or poor, must submit to
the working of this power........
The leaven hidden in the flour works invisibly to bring the
whole mass under its leavening process; so the leaven of truth works
secretly, silently, steadily, to transform the soul. The natural
inclinations are softened and subdued. New thoughts, new feelings, new
motives, are implanted. A new standard of character is set up--the life
of Christ. The mind is changed; the faculties are roused to action
in new lines. Man is not endowed with new faculties, but the
faculties he has are sanctified. The conscience is awakened. We are
endowed with traits of character that enable us to do service for God.
"Faith cometh by hearing, and hearing by the word of God." Rom.
10:17. The Scriptures are the great agency in the transformation of
character. Christ prayed, "Sanctify them through Thy truth; Thy word is
truth." John 17:17. If studied and obeyed, the word of God works in the
heart, subduing every unholy attribute. The Holy Spirit comes to
convict of sin, and the faith that springs up in the heart works by love
to Christ, conforming us in body, soul, and spirit to His own
image. Then God can use us to do His will......
Received into the heart, the leaven of truth will regulate the
desires, purify the thoughts, and sweeten the disposition. It quickens
the faculties of the mind and the energies of the soul. It enlarges
the capacity for feeling, for loving......
The leaven of truth works a change in the whole man, making the
coarse refined, the rough gentle, the selfish generous. By it the
impure are cleansed, washed in the blood of the Lamb. Through its
life-giving power it brings all there is of mind and soul and strength into
harmony with the divine life. Man with his human nature becomes a
partaker of divinity.  COL95-102