Some Traits of Heretics

Ἐισί τινες ἐν ἐνδύμασι προβάτων, ἔσωθεν δέ εἰσι λύκαι ἄρπαγες· οὒς ἀπο τῶν καρπῶν ἐπιγινώσκομεν. Οὐτοι τὲ πρόβατα δοκοῦσι μέν εἶναι ἐξ ἡμῶν, άλλ οὐκ εἰσὶν ἐξ ἡμῶν· εἰ γὰρ ᾐσαν ἐξ ἡμῶν μεμενήκεσαν ἂν μεθ' ἡμῶν. Ὅταν δὲ τὸν ἑαυτῶν ἀπεμέσωσι δυσσέβειαν, τὸν ἐν αὐτοῖς ἀποῥῥηξαντες δόλον, ἐπελαβόμενοι τῶν ἀσθενεστέρων καὶ τὰ αἰσθητήρια πρὸς τὰ θεῖα λόγια ἐχοντων ἀγύμαστα, σύν ἑαυτοῖς ὠθοῦειν αὐτοὺς εἰς ἀπώλειαν, στρεβλοῦντες καὶ σκώπτοντες τὰς τῶν πατέρων διδασκλίας, ὡς καὶ τὰς λοιπὰς γραφάς, πρὸς τὴν ἰδίαν ἀπώλειαν ἑαυτῶν· οὔς δεῖ προγινώσκοντας φυλάστεαθαι, μὴ τῇ πανουργία αὐτῶν ὑπαχθεντες τινές, ἐκπέσωσι τοῦ ἰδίου στηριγμοῦ. Ἠκόνησαν γάρ γλῶσσαν αὐτῶν, ὡσει ὄφεως· ἰος ἀσπιδων ὑπὸ τὰ χειλη αὐτῶν.   Φλαυιανος Ἐπισκοπος Κωνσταντίνου Λέοντι Ῥώμης

There are some in sheep's clothing but inwardly they are ravening wolves whom we know by their fruit. These sheep seem to be of us, but they are not of us, for if they had been of us, they would no doubt have remained with us. 1 But when they have vomited forth their impiety, their deceit bursting out, seizing the weak and those who have senses untrained in the divine words, they drag them off with themselves to destruction, twisting and mocking the teachings of the fathers, and likewise the rest of those things written, also to their own destruction, and so we must be forewarned to be on guard lest some should be misled by their deceptions and shaken in their firmness. For they have sharpened their tongues like serpents, adder's poison is under their lips. 2 Flavian Bishop of Constantinople to Leo of Rome 1 1 Jn 2.19  2 Ps 139.4