Well, you didn't ask me to explain the verse, you asked what leaven was. lol ...
Jesus illustrated the experiences of His Church as an "embryotic Kingdom". I believe this parable demonstrates that. He tells of a woman who hid some leaven in three measures of meal until the whole batch was leavened. This, to me signifies a corrupting of the Good News, the spiritual Food which God had prepared for the Church.
A woman in symbolic language represents a church system; and the
Lord tells us that such a church system will mix leaven, or ferment, in the food of the family of God until the whole mass will be corrupted. It should be noted as I stated earlier that leaven in the Bible is always used as a symbol of corruption, of sin. This parable shows that the faith once delivered to the saints would be lost, mixed with error, until it would no longer be nourishing to the
family.
The Apostle Paul pictures the same matter, saying that in the latter days "some shall depart from the faith, giving heed to seducing spirits and doctrines of demons." 1 Timothy 4:1.
It can be seen that these false doctrines are what are troubling Christians today. Our hearts are better than our heads; for the hearts of the consecrated Christian is in tune with God, while the creeds of the Dark Ages are out of tune. The blessings that many are receiving tody is largely the result of breaking loose from the creeds formulated in the Dark Ages, and getting back to the teachings of Jesus, the Apostles and the Prophets, the only inspired authorities. Their words alone constitute the proper food where we are to be nourished. The Word of God is sufficient, that the man of God may be thoroughly furnished. 2 Timothy 3:16,17.
Some Christian, looking at matters evidently from the wrong angle, are still deceived into thinking that it is possible for the Church, as the Kingdom of God in an embryotic condition, to do the work which God assigns to that Church in her future glorified, perfected condition. They shut their eyes to the fact that the number
of non-christians in proportion to the number of Christians doubles every century. They try to count up Christians by the hundreds of millions, entirely ignoring the fact that Jesus stated that the Gospel Church, the Elect, who are walking in His steps, will be altogether but a Little Flock. Luke 11:32.
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"Pain is inevitable. Suffering is optional."