The leper's public marking as a model for separating from sinners
Origen is commenting on Leviticus 13:45, where the law prescribes how a leper must appear in public, and he connects this to the Apostle's command to avoid sinful brothers.
Verse 45: “And the leper in whom is the plague of leprosy—his garments shall be loosened, and his head uncovered” (Leviticus 13:45). Why does it command that the leper’s head be uncovered and his garments loosened? So that he may be recognizable, and those who approach may not share in his uncleanness. Thus the Apostle said about sinners: “Keep yourselves away from every brother walking disorderly and not according to the tradition which you received from us” (2 Thessalonians 3:6), and, “If anyone named a brother is a fornicator, or greedy, or an idolater, or a drunkard, or a reviler, or a robber—do not eat with such a person” (1 Corinthians 5:11).
Greek
Στίχ. με΄. Καὶ ὁ λεπρὸς, ἐν ᾧ ἐστιν ἀφὴ, τὰ ἱμάτια αὐτοῦ ἔσται παραλελυμένα, καὶ ἡ κεφαλὴ αὐτοῦ ἀκάλυπτος.
Διατί τοῦ λεπροῦ ἀκάλυπτον εἶναι κελεύει τὴν κεφαλὴν, καὶ τὰ ἱμάτια παραλελυμένα; Ἵνα γνωριστὸς ᾗ καὶ μὴ μετσλαγχάνωσι τῆς ἀκαθαρσίας οἱ προσπελάζοντες. Οὕτως ὁ Ἀπόστολος περὶ τῶν ἁμαρτανόντων ἔφη· Στέλλεσθε ὑμᾶς ἀπὸ παντὸς ἀδελφοῦ ἀτάκτως περιπατοῦντος, καὶ μὴ κατὰ τὴν παράδοσιν, ἣν παρελάβετε παῤ ἡμῶν· καὶ, Ἐάν τις ἀδελφὸς ὀνομαζόμενος, ἢ πόρνος, ἢ πλεονέκτης, ἢ εἰδωλολάτρης, ἢ μέθυσος, ἢ λοίδορος, ἢ ἅρπαξ· τῷ τοιούτῳ μὴ συνεσθίειν (18).