(57) To teach when it is unclean.--This verse is intimately connected with Leviticus 14:54, viz.: "This is the law for all manner of plague of leprosy . . . to afford instruction in the day of uncleanness, and in the day of cleanness:" that is, to furnish them with instructions against the time when they would have to deal with these cases in taking possession of the promised land. The ancient authorities, however, insisted upon the literal rendering which is substantially exhibited in the Margin of the Authorised Version, viz., "To teach concerning the day of uncleanness and concerning the day of cleanness: i.e., to instruct the people on which days this distemper may be examined and decided. Hence the ancient Chaldee Version of Jonathan renders it, "That the priests may teach the people to discern between the dark days, when his leprosy is to be examined, and between the bright days." (See Leviticus 13:2.)

14:54-57 When that God who is rich in mercy, for his great love wherewith he loved us, even when we were dead in sins, hath quickened us by his grace, Eph 2:4,5, we shall manifest the change by repenting, and forsaking former sins. Let us follow after holiness, and let us compassionate other poor lepers, and desire, seek, and pray for their cleansing.To teach when it is unclean, and when it is clean,.... A man, his garment, or his house; for it respects them all, as Aben Ezra observes; which was the business of the priests to teach men, and they by the above laws and rules were instructed how to judge of cases, and by which they were capable of pronouncing persons or things clean or unclean:

this is the law of leprosy; respecting every sort of it, and which is very remarkably enlarged upon.