In the Gospel Jesus said, “Beware of the leaven that is the hypocrisy of the Pharisees.” In other places Jesus used leaven, as a metaphor for something very fine: for his kingdom.
Leaven, or yeast, is a fungus: something that is living. Only 1% of fungi are yeasts; but there are 1500 known varieties of yeast-fungi. (Korean country people used to raise their own yeast with bags of soggy beans they let hang out in the weather.) As a yeast fungus reproduces asexually releasing carbon dioxide which in bread dough creates empty pockets that swell the dough.
In the case of some, but not all, Pharisees, their pride was like a yeast that puffed them up. In the case of sincere Christians the good example they give hour by hour in their workplaces raises an appreciation for goodness among all with whom they work. That was what Jesus meant when he said, “The kingdom of heaven is like yeast which a woman takes and mixes with three measures of flour until the whole mass begins to rise.”
It is that kind of yeast that Phyllis McGinley had in mind when she wrote that St. Jerome “Filled the world with a Christian leaven.”