The Fourth Sentence in the Letter to Theodore

Πεφυσιωμένοι γὰρ εἰς γνῶσιν, ὡς λέγουσι, τῶν βαθέων τοῦ Σατανᾶ λανθάνουσιν εἰς τὸν ζόφον τοῦ σκότους τοῦ ψεύδους ἑαυτοὺς ἀποῤῥίπτοντες καὶ καυχώμενοι ἐλευθέρους εἶναι δοῦλοι γεγόνασιν ἀνδραποδώδων ἐπιθυμιῶν. 

λέγουσι [MS, PV] λέγουσιν [AT]

Tselikas's transcription makes more sense. In the first sentence: "Πεφυσιωμένοι γὰρ εἰς γνῶσιν, ὡς λέγουσιν," the form "λέγουσιν" is used. This form is the third person plural, which agrees with a plural subject. In the second sentence: "Πεφυσιωμένοι γὰρ εἰς γνῶσιν, ὡς λέγουσι," the form "λέγουσι" is used. This form is also the third person plural but in a different grammatical form. Both forms are correct in their respective contexts. The difference lies in the emphasis or style of the writing. The first one ("λέγουσιν") is more formal or classical, while the second one ("λέγουσι") is more commonly used in modern Greek.