The 13th chapter of Matthew is both a fork in the road and a litness test for the believer in Christ. Are you going to read the Bible with Jewish or Gentile definations? What is your interpertative stance? Do you read it as you always have or do you read it as it is?
Like all definations and types in the Bible, the must be taken in context and remain consistant with any usages in previous passages. Leaven is always used as a type of sin. To a Jew, putting leaven in the bread (the reference is the fellowship offering in the book of Leviticus) is a very bad thing. (In the book of Genesis, Sarah is asked by Abraham to make a fellowship offering for Lord and two angels and to use unleavened bread.)
This scripture is clear. In the kingdom of heaven there is sin that permeates the kingdom and is found everywhere. Now before you think that I am spreading heresy . . . just what is the kingdom of heaven anyway? Is it just a place of the saved or is it more than that.
Follow my logic. Both Jesus and John the Baptist began preaching, "Repent. The kingdom of heaven is at hand." They do not define this "kingdom" but just proclaim it. (It is from our New Testament only theology that we draw the conclusion that the kingdom of heaven is just the rehealm of the saved. In the Old Testament, the kingdom and the temple are always two separate entinitys. Salvation is appropriated at the temple. Justice and judgment is found at the throne. The law is appeased at the temple. The law is fulfilled at the throne.
The kingdom of heaven is the economy of God, not the people of the kingdom. It is how Christ will rule, not those who are ruled. It is Him, not us. We enter the kingdom by making Christ our King. (This is not Lordship salvation. Our salvation is by Christ our High Priest. Our purposes and placements are from our King. The kingdom of heaven exsists now in the spirit but one day it will be both in the spirit and in the flesh. Jesus will come back and rule from His throne in Jerusalem for a thousand years. Many of these parables in Matthew 13 are about His Millienial Reign. It is this Millienial Reign that will begin with the second coming of Christ and end with a war with the people of the earth who reject Him and follow Satan when he is loosed for a while. There will be fleshly people in the Millienial Kingdom and it is these carnal people and their pride that will be the sin that perminates the kingdom of heaven.
When you study the term "kingdom" from Moses thru David and the split kingdom, you will find that "kingdom" is not about who is being governed but how the people are governed. After His second coming, Christ will reign over the whole earth with a rod of iron but many will reject and hate this and thus be the unleaven bread that permeates the loaf.
I hope that this brief explanation is adequate. We are all woefully ignorant of the millienial reign. (Many popular end-times books and movies and teachers stop just after the second coming and do not realize that there is still an important chapter to go.
Write me if this is vague or confusing.