Luke 17:13-14

Luke 17:13-14. And they lifted up their voices, and said, Jesus, Master, have mercy on us. And when he saw them, he said unto them, Go shew yourselves unto the priests.

For no man could be pronounced clean even if he were healed, until he had undergone the ceremony prescribed in the Mosaic law. These lepers were to go to the priests just as they were, so their going was an act of faith.