"The Kingdom of Heaven is like the yeast a woman took and mixed in with three measures of flour till it was leavened all through" - Matthew 13:33
Why would our Lord use the image of yeast to describe the Kingdom of heaven? Yeast are very curious creatures - actually they are eukaryotic microorganisms, meaning they are really fungi. However yeast are a brewer's best friend, allowing us to convert monosaccharide sugars into alcohol and carbon dioxide. They also release other by-products depending on the strain that help to give beer its distinctive flavors.
The core of Jesus' preaching message was the coming of the Kingdom of Heaven. Yet the Kingdom is not only some distant reward that we will receive upon death. Christ's coming ushered in the Kingdom of Heaven in the present! As St. Paul reminded the Corinthians:
"Behold, now is a very acceptable time; behold, now is the day of salvation." - 2 Corinthians 6:2 (Emphasis added)
So what does the Kingdom of Heaven require of us? I believe our Lord gives us the answer in his image of the yeast. Just as the yeast converts the wort (unfermented beer that is undrinkable) into beer that we can enjoy, the Kingdom of Heaven requires us to introduce spiritual yeast into our own lives to begin the process of conversion (in Greek, metanoia). If we allow Christ to permeate our entire being, if we allow Him to love us as we were meant to be loved, we will be able to participate more fully in His divine life both in the Kingdom here, and in the heavenly Kingdom to come.
As we begin this season of Advent, ask God for the grace to be open to metanoia, to conversion. Allow God's love to be a leaven, transforming your heart as you prepare for Christ's coming at Christmas.