As soon as Jesus descends from the mountain after his sermon, a leper comes to him and asks to be healed. A leper is someone whose diseased skin makes him unclean, who is banished from Israelite society for the duration of his uncleanness. In the Old Testament the clean vs. unclean distinction is meant to instruct the Israelites about God's holiness. What is clean may come into his presence and what is unclean may not. So when we see this leper, we are supposed to see ourselves. Our sin makes us spiritual lepers. As long as we remain unclean we are banished from the presence of God. And so like this man in our passage, we come to Jesus with hesitation. "Lord, if you are willing, you can make me clean."
A lot of us share this leper's mixture of faith and doubt. We don't doubt Jesus' power to make us clean. We're certain about his capabilities, his ability to do wondrous works. He created the world. He is all-powerful and all-knowing. We know he has come to be the Savior of the world. We just aren't sure if he has come to save us in particular. "Lord, if you are willing, you can make me clean." I know you can make me clean, but do you want to?
But Jesus responds even to this very weak attempt at faith. It's a faith that says, "Yes, Lord, I do have faith in a theoretical Jesus that is perhaps only a product of my wishful thinking, who would cleanse me if he were the type of person who cared enough to have compassion on me." It is not even full faith, really. It's just a sorry scrap of faith that this broken and discouraged man tosses out, half expecting to be turned away.
That's why Jesus knows he has to do more than just zap the leprosy away. He reaches out and touches the man, an act that would have defiled anyone else, and says, "I am willing. Be cleansed." He knows full well that this is someone who has been starved of the comfort of human touch. It is just as important to Jesus to assure him of his love as it is to grant him healing. And instead of being defiled by the man's uncleanness, Jesus "defiles" the man with his own cleanness. He is not the kind of holy man who shrinks to one side of the road so as not to be dirtied by the Samaritan lying half dead on the other side. He rather invades our space with his cleansing touch, if only we'd stand there and let him. And as he told the leper, he is more than willing to infect us with his wholeness and health.
Why does he instruct the healed leper to be silent about his miraculous healing and instead see a priest? In some cases Jesus asks people to keep quiet about a healing because he doesn't want to be mobbed by a bunch of thrill seekers. But Jesus has been fairly public about his healings (4:24-25) in this particular region, so I think the purpose of ordering the man to see a priest is to have the truth of his healing confirmed through more official channels, rather than trying to make a bald claim. The priest would have to go through the process laid out by the law of Moses: checking the man's skin, pronouncing him clean, making an offering of two birds, isolating the man for seven days, and finally having the man come forward to present a guilt offering, a sin offering, a burnt offering and a grain offering for atonement (Leviticus 14:1-32).
The Pharisees and religious leaders are sure to try to discredit Jesus' healing by challenging this man's right to be accepted back into Israelite society. "He's not really clean! This Jesus sent a leper back in our midst to defile us all!" So when Jesus tells the man, "Go, show yourself to the priest and present the offering that Moses commanded, for a testimony to them," he may be trying to protect this guy. He's asking him to jump through all the hoops so that he can be officially pronounced as cleansed, to testify to the truth of Jesus' healing power and of the man's right to be accepted back into his community.