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The brief parable of the leaven is preserved by the Evangelists Matthew (13:33 35) and Luke (13:20 21). The Evangelist Matthew has it thus:
"The kingdom of heaven is like unto leaven, which a woman took, and hid in three measures of meal, till the whole was leavened."
Here Christ compares the Kingdom of Heaven not to leaven itself, but to its action on flour and dough. The Lord chooses analogy to a natural process to emphasize that His word is changeless like the laws of nature. Christ's trace of leaven [yeast] in a large volume of dough can stimulate fermentation, like the inner, hidden action of Gospel preaching can quicken the world and human hearts.
Apostle Paul writes: "Know ye not that a little leaven leaveneth the whole lump?" (I Corinthians 5:6). Leaven, no matter how little, imparts acidity to the whole mixture of flour. The living, creative action of the Kingdom of God is likened to dough, raised by yeast. The heavenly leaven - the Divine Spirit - placed by the Savior in human souls quickens the Kingdom of God on earth, so that its true children can emerge.
According to the Serbian bishop, Nicholas of Ohrid, in the Kingdom of God, the three measures of the parable may signify the three branches of the human race: Semites, Japhites, and Hamites. The Savior brought heavenly leaven or grace to all without limit. According to Blessed Augustine, the three measures of flour signify the three main powers of the human soul: mind, heart and will, or the three powers of thought, feeling, and operation, gradually sanctified by the Holy Spirit. The grace of God penetrates and sanctifies the spirit, soul, and body of man. When armed with the power of grace, a man enters a new, Christian life. His mind becomes the mind of Christ, fully obedient to faith and capable of attaining the Mystery of Salvation. The desire and actions of a man rise to full agreement with the will of the Lord. In the heart of a man reborn and cleansed by grace, the peace of God reigns. The body of a reborn man itself becomes a pure vessel of pure faith.
The Fathers of the Church emphasize the purity: the heavenly leaven acts on dough made of unspoiled flour. Stale, spoiled flour will not turn sour, rise, nor yield to the action of yeast, just as the grace of God does not act in a negligent soul. For Divine leaven to do its work, we must repent and struggle with all vices, and prepare the dough of our soul and body in patience. Success in the struggle with one's selfishness and with one's "old man" is easy or quick. Every minute, every day, is a struggle with temptation and sin, in order to live as the Lord wishes.
Every conversion to Christianity begins with something small, as small as a mustard seed or a speck of leaven. We take this gift from Christ's Church. The closer and more sincerely we unite ourselves to Christ in this mystery, the more spiritual strength we shall gain to fulfill His commandments.