Over the course of history, symbols change their meaning. How could they not? Just as languages morph and change because people use them, so do things that represent certain ideas, institutions, establishments etc. The swastika is an ancient symbol from India that means (meant) something like peace or well being. That is, until that goose-stepping, blot-mustached Nazi changed its meaning forever. If I place a swastika crocheted in sky-blue thread into a frame on the wall of my hallway with the inscription “Love All People” beneath it, anyone aware of what happened in the 20th century will see the irony and the misplacement of the symbol.
Symbols in the Bible work much the same way, changing over time as the Author develops the story. Not all symbols are apt to change into their opposites. For example, contra Shrek, the dragon remains that serpent of old from Genesis to Revelation. But other things take on new meanings, and in order to understand what the text means, we can’t confine a later meaning to an earlier one.
In the Passover, Israel had to get leaven out of their houses and eat unleavened bread for seven days (Ex. 12:15). The leaven is the leaven of Egypt, and get rid of it meant, among other things, leaving Egypt behind. Paul cites this when he tells the Corinthians to keep the New Covenant Passover by ridding their hearts of the leaven of malice and wickedness (1 Cor. 5:8). In this sense, leaven is still bad and always will be.
But Jesus also added a new layer to the symbol of leaven when he said the kingdom is like yeast, working through the lump. Those who see the world as going from bad to worse have to misconstrue this metaphor, somehow turning the meaning on its head and insist the leaven is evil when Jesus is plainly talking about the sure growth of his kingdom, slow though it is.
Few churches serve unleavened bread in communion, but perhaps even fewer serve a leavened lump knowing what it means. Even if they do on an abstract level, the routine practice of making the supper a time of sadness or overly-solemn reflection on sin flies in the face of the meaning of the new leaven. Communion is a foretaste of the wedding supper of the Lamb, and as often as we eat and drink it, we proclaim the Lord’s death until he comes, a proclamation that is more about the death of sin than a lament over its remaining effects. We still rid our hearts of the leaven of sin, but we do this because the leaven of Christ’s kingdom has conquered and is conquering. Communion should feel more like a feast than a spiritual fast, like a celebratory meal with God of a gospel-leavened people.