John Lightfoot on Luke 5:12

[When he was in a certain city, behold, a man full of leprosy.] "The walled cities are more holy than the land of Israel in general, because they cast out the leprous from them." Which must be understood (if we allow of the Rabbins for interpreters) of cities that had been walled from the days of Joshua. If this city which the evangelist here mentions were of that number, no leper would have been suffered in it, unless absolved from his uncleanness by the priest. For the leprosy remained after that absolution; and the sick man was not healed but restored to the church. That the man is here said to be full of leprosy; the passage may not impertinently be compared with Leviticus 13:12,13.

Whether he had been purified by the priest before or no, however, Christ sends him to the priest, to offer what was required from the leper that was cleansed. The law of Moses hardly supposeth the leper healed when he was made clean. It is a question, indeed, whether the disease was ever curable but by a miracle. And therefore is this man sent to the Temple to shew himself to the priest, and offer for a testimony unto them, verse 14: that is, that he might bear witness, that the leprosy, an incurable disease, was now healed by miracle, as formerly it had been in Miriam and Naaman: and so there was now a great prophet arisen in Israel.