"He has come and mercifully taught us in such a way that we may walk as children of the light and be true children of the light. So the Christian life is supposed to be luminescent, like the lights on a landing strip at an airport on a foggy night that help planes land," he said.
"Similarly, light gives off warmth, and Christ has come into the world to warm us by his love, to burn away whatever in us is frigid or tepid, so that we in turn may warm others by the fire of divine love."
Leaven is an important metaphor, Landry said, because it teaches us that just a small number of devout Christians can have a significant impact on the world.
"We know that leaven works unseen, doesn't call attention to itself, but it does its work all the same. Much of the most important work of Christians happens through example - by our cheerfulness, by the powerful transforming influence of good friendship, by the encouraging smiles and deeds that others need when they are down," he said.
He pointed to the example of the early Japanese Christians, who kept the Catholic faith alive for centuries after the priests in the country had all been martyred.
Missionaries in the 1800s found hundreds of Catholics in a village in northeastern Japan. In place of Mass, the Catholic families would gather every Sunday to pray the Apostles' Creed, Our Father, Hail Mary, Glory Be, and an Act of Contrition.
The villagers had been instructed by previous missionaries to be steadfast and to wait for more "fathers" to come after the initial missionaries were martyred. The early priests had taught the people how to recognize the four marks of the Catholic Church: belief in the real presence of Christ in the Eucharist, devotion to Mary, the papacy ,and the priesthood.
"So great was the trust in God and in his Church by those who were being killed in the 1600s that they prepared the people for the time when Catholic priests would return to Japan, and their simple instructions were passed down by the kakure Kiristan, the clandestine Christians, for a dozen generations," Landry said.