Leviticus 13:3

The priest shall - pronounce him unclean - וטמא אתו vetimme otho; literally, shall pollute him, i. e., in the Hebrew idiom, shall declare or pronounce him polluted; and in Leviticus 13:23, it is said, the priest shall pronounce him clean, וטהרו הכהן vetiharo haccohen, the priest shall cleanse him, i. e., declare him clean. In this phrase we have the proper meaning of Matthew 16:19 : Whatsoever ye bind on earth, shall be bound in heaven; and whatsoever ye loose on earth shall be loosed in heaven. By which our Lord intimates that the disciples, from having the keys, i. e., the true knowledge of the doctrine, of the kingdom of heaven, should, from particular evidences, be at all times able to distinguish between the clean and the unclean, the sincere and the hypocrite; and pronounce a judgment as infallible as the priest did in the case of the leprosy, from the tokens already specified. And as this binding and loosing, or pronouncing fit or unfit for fellowship with the members of Christ, must in the case of the disciples be always according to the doctrine of the kingdom of heaven, the sentence should be considered as proceeding immediately from thence, and consequently as Divinely ratified. The priest polluted or cleansed, i. e., declared the man clean or unclean, according to signs well known and infallible. The disciples or ministers of Christ bind or loose, declare to be fit or unfit for Church fellowship, according to unequivocal evidences of innocence or guilt. In the former case, the priest declared the person fit or unfit for civil society; in the latter, the ministers of Christ declare the person against whom the suspicion of guilt is laid, fit or unfit for continued association with the Church of God. The office was the same in both, a declaration of the truth, not from any power that they possessed of cleansing or polluting, of binding or of loosing, but by the knowledge they gained from the infallible signs and evidences produced on the respective cases.

The hair in the plague is turned white - The sparing growth of very fine whitish hair on leprous spots in the place of the natural hair, appears to have been always regarded as a characteristic symptom.

the plague in sight be deeper than the skin of his flesh - Rather The stroke appears to be deeper than the scarf skin. The bright spot changed to a brownish color with a metallic or oily luster, and with a clearly-defined edge. This symptom, along with the whitish hair, at once decided the case to be one of leprosy.

13:3 When the hair is turned white - This change of colour was an evidence both of the abundance of excrementious humours, and of the weakness of nature, as we see in old and sick persons. His flesh - For the leprosy consumed both the skin and the flesh.